Depends on a lot of things including his physiology, his mental state, how recently and frequently he has had sex/masturbated, etc.
Largely, it depends on how sensitive he is down there. Some guys are really sensitve and really have to sweat to last more than 30 seconds of constant pumping, other guys I know say they aren't sensitive enough and they have trouble finishing at all -- they go for 45 minutes straight and can still never get there.
It can also vary with one individual, sometimes a guy can last much longer on one occasion than he did on another. It all depends.
What I find to be a real shame is how much pressure women put on guys to last a certain amount of time or please them in a certain way, and they expect their man to satisfy them first and foremost. It's all take and no give -- when in fact the best solution would be to HELP him find a way to last longer and treat him like a teammate rather than a coin-operated dildo machine. Lovemaking should be approached as a two-way street, not a selfish act of "okay, it's your job to please me" personal gratification.
EDIT: after reading some of the answers, seems like a lot of women have a misconception that guys can willfully control when they bust it, and they "choose" to come early or hold it off. It's not that easy. You pretty much come when you come and you can't control it, unless you stop for a breather.