I would not reuse that oil. The frying-oil reuse that occurs in fast food chains is reuse for the same day or over just a few days. They also store it properly and not just sitting exposed in the fryer.
justkt is correct that more saturated fats are less sensitive to breakdown, this is why bacon grease can be kept for a rather long time.
Three factors play a significant role in oil degradation:
Oxidation
Any contact with air causes oxidation in oil. High temperatures, metal alloys, surface exposure, and even UV light act as catalysts to this reaction.
Hydrolysis
When water interacts with oil it causes it to taste tainted or acidic. This is exacerbated by high temperatures, heating/cooling cycles, and oxidation products.
Polymerizaton
When frying oil deteriorates, the resulting products form both volatile (or reactive) and non-volatile compounds. Non-volatile compounds remain within the frying oil, and can produce polymerization at frying oil temperatures above 200°C (390 F) or in isolated hot spots within the frying system. These molecules bond together to form large, different-sized clusters that accumulate on the oil’s surface. Since they don’t dissolve, they cause foaming; trapping air under the oil, and increase the possibility of hydrolysis.
Given all these, the best environment for any oil is an cold, dark, airtight place. Cold temperatures may cause clouding, but this is not a cause for worry.
Source:
http://www.heatandcontrol.com/technical%20articles/Maximizing%20Cooking%20Oil%20Life.pdf
For the very best tasting fries, onion rings and battered fish are fried in fat made from rendered beef fat. When I was a cook we rendered down a thousand pounds of beef fat a week, it took days to do. But it made the very best tasting savory deep fried foods. The burning temperature is lowish, so food needs to be cooked at 325 and changed more often. It's not good for things like doughnuts.
I'm not a fan of Canola, I find it often has a taste which I don't like. I don't use peanut mostly due to habit, too many people I've run into with allergies. I can't find rendered beef fat and I'm not going to render it at home.
At home I use a mixture of Sunflower and lard, about 1 part to 3 parts. The sunflower oil makes it easier to handle when filtering through a very fine mesh sieve (more liquid at a lower temperature) after use, then I put it in an old olive oil can which I refrigerate between uses. I leave out overnight before I use it then warm in hot water to pour it out. I don't usually use more then once a month. The more crumbs and batter bits you filter out the longer the fat will remain good. So filter, and don't pour the last bit into the can.
Refrigerated the oil mix doesn't go bad before it becomes no good to use. I get well over a years use. I love sunflower oil, it has almost no taste at all and a high smoke temp. It is pricey.
Best Answer
If you strain the oil using a coffee filter inside a funnel, it will catch even the tiny bits that would slip through a wire strainer, making your oil super-clean. Store in the refrigerator, ideally in a glass bottle or jar, and it will keep for at least six months. I haven't stored it undisturbed for longer than that, so I can't speak to how well it keeps past that point. If you use and then re-filter the oil, you can start the six-month count over. I have only repeated this twice, so again, I can't speak to how the oil keeps when you do this many times.
One more item: I don't know if it matters, but I use grapeseed oil. I would assume that this also works for other frying/deep-frying oils, but I can't say for sure.