Well, the easy answer is "until it's done". But that's not the whole answer! It really depends on what your plan is for the beer. Are you using a secondary (clearing tank)? If you are, I like the 1-2-3 method- that is, 1 week in the fermenter, 2 weeks in the secondary, 3 weeks in bottles then drink!
If you're not using a secondary fermenter, I like to leave my beers in the fermenter 2-3 weeks, and until the sg is stable three days in a row.
Welcome to HBT! Check out the stickies and ask any questions you'd like. It's nice to have you here.
The best advice you can get right now is to read EVERYTHING you can find. The search option for the forum is a great tool. Yes, good luck and enjoy!:rockin: