It all really depends on the guy and the girl he's having sex with. Generally, I've found that if it's with a girl that I really car about, I can go a long time (gone 45 minutes once, sore the next day
, avg 10 - 30min), but if it's someone you don't really care about, it's shorter. It also depends on how much foreplay you do. If you do a LOT of foreplay, then you'll be a lot hornier than if you jumped right into it, making it last an even shorter amount of time.