This question involves a lot of sub-questions that don't have simple answers, so this response is a bit long.
From my experience producing dozens of starters from scratch over the years, I have to disagree with some of the details given in the other answer. There are two goals in the early days of a starter:
- Produce an environment hospitable to the growth of yeast and lactic acid bacteria, while discouraging growth of other nasty things that could take over your starter and cause it spoil.
- Begin producing a "rhythm" of feeding that will encourage the natural cycle of yeast and bacteria growth. Generally speaking, yeast will grow quickly after a feeding (the rising phase), but their waste products and other changes will allow bacteria to take over and start growing strongly afterward (the "sour" phase). If the culture gets too sour before the yeast are well-established, and feeding schedules are infrequent, it can take weeks to establish a starter (if ever). If a culture isn't sour enough, bad things may begin to grow and spoil your starter. The trick is finding a middle way, and a feeding schedule is a major part of that.
(1) Feeding your starter more than once per day is, in my opinion, a useful tool for guaranteeing a high success rate in the first few days of a new starter. At a minimum stirring at least a couple times per day will inhibit mold and other microorganisms from getting established on the surface before the culture becomes sour (i.e., acidic). It is true that feeding too frequently may not allow appropriate levels of sourness to occur in the first few days, but this can be overcome by two methods.
One solution is to add artificial acidity. After many, many trials, various forums online have discovered pineapple juice is a near-ideal addition to bring down pH and add acidity in the first 2-3 days of starter growth. Use it instead of water. (Note that this process can be done with feedings every 24 hours or more frequently.)
The other solution is to only add partial feedings for the first couple of days (for example, following a regimen like this one). To explain this method in more detail, rather than doubling the starter with each feeding (and discarding half), you would add the same amount each time for the first few feedings without discarding anything. Thus, the first feeding would double the quantity of starter, but the second would only add 50%, the third would add 33%, the fourth 25%. Each addition will provide fresh food to encourage yeast growth, while simultaneously concentrating the acidity.
(2) The "float test" is good for mature starters, but I don't think it's good enough to assess when your starter may be ready for bread. A very strong starter will take off and rise significantly within a few hours after feeding. I would wait until this happens.
(3) First, there is no reason to be wasteful. I have no idea why many starter recipes start with a cup of flour or whatever and waste pounds of flour in the first week. I've often started with 10-20 grams of flour. And then I follow a regimen like the second link above or something like suggested in the second page of discussion on the pineapple juice solution, where you only need a few tablespoons to get going and you don't need to discard anything for at least the first few days. Working in small quantities won't delay the establishment of your starter, and if you're only using a tablespoon or two of flour for each feeding, it's not a lot to discard.
As for ratios for feeding, as I said, many prefer a solution that actually doesn't discard anything for the first few feedings to produce adequate acidity. After you start seeing a yeast response with lots of rise (not just small bubbles which may be due to bacteria activity or the large surge in bubbles which usually happens in the first 48 hours, due to bad bacteria activity before the culture becomes sour), you can begin to discard and replenish. Many people double the culture with each feeding at this point, but I've had better results doing a 1:1:1 feeding (1 part starter to 1 part flour to 1 part water by weight) at this stage, as long as the yeast are clearly showing some activity, even if they might be slower.
Once the starter is mature, you can be more aggressive with discarding and replenishing, or alternatively planning ahead and only saving small quantities of starter for the next batch, so you don't have to discard much at all. I find the 1:1:1 feeding strikes a good balance (effectively equivalent to discarding 2/3 of the starter and replenishing the rest). Saving smaller amounts of starter will tend to produce a more mild starter, while doing your method of discarding half can lead to a somewhat more sour product, which some people prefer. Doing a 1:2:2 feeding or even using smaller quantities of starter at each feeding can also have benefits, but it requires a strong, mature starter. (For some discussion of various opinions on feeding ratios, see here. In general, the right ratio is what seems to work with your starter and to produce bread you think tastes good.)
(4) You can get away with a 24-hour period between feedings for the first few days, but as I said, I'd at least stir it every 12 hours or more frequently to avoid mold settling. The pineapple juice links above recommend against feeding more frequently in the first couple days to avoid diluting the acid. I agree with that advice if you were using water and discarding and doubling the starter with each feeding, since you wouldn't be allowing proper acid development. But if you follow the juice method or the other method outlined in the links, the acidity will become concentrated with the initial feedings, rather than diluted, and more frequent feedings can allow the yeast to "catch on" quicker.
Once established (and the 12-hour rise-and-fall cycle is a good benchmark), I would definitely recommend a 12-hour feeding cycle when sourdough cultures are kept at room temperature, unless you prefer a more sour bread result (and potentially a weaker culture). It really depends on your room temperature, your feeding proportions, and a number of other things -- sourdough can work with a lot of different conditions. I tend to use my sourdough for a variety of breads, and I rarely actually use it to make very sour "sourdough" bread, so I like to keep it very mild-flavored for adaptability reasons. Feeding every 12 hours at room temperature (or even every 8 hours in warm temperatures) will do that. When I'm not baking for a while, I put the culture in the fridge and take it out to do at least one feeding before reviving.
Again, a little thought can avoid waste. There is no reason to keep cups and cups of starter lying around, discarding half every time you feed it. When I save culture in the fridge, I generally save about 1/9th of what I actually need for my average batch of bread (which may only be a couple tablespoons: I often keep my starter in a tiny jar left over from Dijon mustard or something). Then I remove it the day before, do a 1:1:1 feeding, 12 hours later do a 1:1:1 feeding (not discarding anything, so it's now 9 times the original starter quantity), and I have a very mild and active starter ready to go. If I haven't used it in a while, I'll do a 1:2:2 feeding or even higher ratio to dilute the excess acidity that tends to build up.
To answer the final question of how long it takes to develop a reliable starter: I'd say with a 12-hour feeding schedule following a regimen like the one in the second link above, I can have a somewhat reliable starter able to bake bread in about 3 days, assuming good quality rye flour. Depending on conditions, it can sometimes take another day or two, but a couple more days of regular feeding regardless will ensure it's really strong. I've done this method over a dozen times, and I've never had a failure. Before that, using a 24-hour feeding cycle, I've had multiple failures (by which I mean mold or other things happened), and it would usually take at least a week to establish a culture. I haven't actually used the pineapple juice method (only water), so perhaps it is more reliable using a 24-hour feeding cycle in the first few days.
(Note that the reason I say I've produced dozens of starters over the years is because I've found these methods so reliable and not wasteful that I often don't even bother reviving an old starter I've neglected for months -- it's almost as easy to make a new one.)
Rye flour, I've found, is really useful, even if you eventually plan to switch to wheat for normal feedings (which you can do once you get the rise and fall in a 12-hour window). With whole-wheat flour, I've often had starters take much longer to get established.
Trying to answer as best I can: Your culture won't smell like alcohol and it won't smell like bread yeast. It will have it's own unique sourdough smell.
Yeast gives off alcohol as part of the process. Different yeasts have different tolerances for how much alcohol they can stand. Basically once the alcohol gets to a certain concentration, the yeast will go dormant. Similar mechanics with beer.
The fact that your cultures doubled is good news. But remember, once the yeast exhausts the available sugars in the flour, they will stop bubbling. That's normal.
If you are leaving your cultures out at room temp, you'll exhaust those sugars quickly.
When I refresh my culture, I leave it out on the counter until it's bubbling and has almost overflowed the container. At that point it goes into the fridge. I will leave it there up to a month before doing another refresh.
Hard to explain but your culture should have a sourdough smell. I can't think of anything to compare it to. Using supermarket sourdough for this may be an exercise in futility as those breads can be pretty lame. My guess is there's nothing wrong with yours.
The only thing I can think that's different from what I do is leaving it out at room temp for more than half a day.
You can abuse sourdough starter up to a point but once you say you're sorry it's your friend again.
Time to make some bread?!
Best Answer
Don't do this. A sourdough starter contains several strains of yeast and bacteria in a fairly delicate balance. These consume sugars and produce CO2 and a range of byproducts. Commercial yeast is a different species of yeast, engineered to eat and reproduce much faster than any of the wild yeasts in your starter. Adding commercial yeast to a starter will lead to the commercial yeast outperforming the wild yeasts and gradually replacing them. You will end up with a commercial yeast culture, rather than a sourdough culture.
Edit I realise I have somewhat misunderstood your question. The answer still holds, though: you cannot start with commercial yeast and expect to create a sourdough starter. The commercial yeast would prevent any wild yeasts and bacteria from colonising the starter. You can keep a culture of wild yeast (to some extent), but as @Chris H's comment states, it won't be sourdough.
If you want a better guarantee of success, your best bet is to somehow obtain an offshoot of someone else's starter. Some bakeries will be happy to give/sell you some, or you can try buying a starter online.