I think the biggest thing to consider is the surrounding terrain. If you have some kind of food source (rice field, cattle, etc), then I think that building a worker would be the best move. This allows you to have a solid food supply, and it will improve the build time of a subsequent settler.
If you don't have a favorable location, or if you're on the coast (where you can work the water tiles for food), then I think a settler would be a better choice.
You really have to weigh up the expansion of your empire vs the surrounding terrain. Sometimes it's conducive to quick expansion, sometimes you'll have to consolidate first.
There are several main factors as to how long Civ IV games last.
1) Game speed setting
The difference between quick (everything is 33% faster build/research) and marathon (everything is 300% longer to build / research) also manifests as an actual increase in total number of turns. (I think "normal" defaults to 500 turns?)
2) Game Map Size
Map size can change the length of victories immensely. Small maps favor conquest or domination victories. Larger maps (with the chance for more science producing cities) lend themselves to (marginally) faster space and Cultural victories.
3) Victory Settings
As mentioned by Marco elsewhere in this topic, there are several different victory conditions. Some of these (Conquest / Religious / Time) can be reached markedly sooner than others (Cultural, Space Race, Diplomatic)
4) How you play the game
Do you automate everything and press "enter" to skip turn after turn? Are you a conquest-minded gamer who needs to micromanage large armies as you expand your empire? There's no easy way to determine how much time a single turn will take.
The bottom line is that a standard Civ IV game will happen over hours, if not days. (Though you may quit at any time, receiving a "Time" "Victory")
Edit: Added "Game Map" as suggested.
Best Answer
It doesn't do anything blatantly stupid, and develops your lands so they'll look a lot like the AI's lands.
What it won't do automatically is city specialisation. It's a powerful strategy to have commerce cities that build no troops just lots of markets and banks, science cities that build lots of libraries and science buildings, and military cities that just build barracks and then crank out unit after unit after unit. The first two kind of cities need their lands developed to make commerce, some food, some hammers. The military city needs no commerce, at least as long as you don't need to turn commerce into culture, but benefits from hammers and enough food to use them all. Then there are the Great People farms that have lots of food and use it to run many specialists.
The advantage is that you save resources by only constructing buildings that make a large contribution to your economy. A handful of markets in the right cities can do more for your economy than building markets everywhere that all earn mediocre amounts of gold.
Each kind of city has a different development needs. Some of that comes from careful selection of city sites, but some of it from the micromanagement of your workers.