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.