It really doesn't matter as long as everyone gets theirs. It can vary, too. Obviously a quickie doesn't last long, but a night set aside for romantic time can last forever. I would say that, with foreplay, a good twenty minutes is fine.
And yes, there is such a thing as too long. I once dated a guy who never went under an hour. After an hour of sex, I'm getting tired, sore, and turned-off. It's not always bad, but when it is like that every time? Too much.
15 minuetes is good!
Lasts? as in how long before he comes during intercourse or how long the sex goes one? Was a time they were the same thing and there was much frustration. Marathon boning generally leads to boredom and chapping so I tend to favor overall time spent naked as a better indicator of time spent.
for us, it's anywhere between 10-25 minutes.
According to Masters and Johnson, the average time for guys from penetration to orgasm is 7.3 minutes.