Sorry if my answer was vague, but it all really depends upon the situation. If she's really getting off, I may not last but a couple of minutes. The more turned on she gets, the more turned on I get, and it becomes absolutely impossible to control. If the sex is pedestrian, "oh all right...let's just get it over with" sex...it can go on for 45 minutes. Ah, sweet irony.
My point was that what you do is far more important than for how long.
How you doin'?
Seriously, I'm no sexual conquistador. I can barely get to my second hand counting the number of women I've been with. I've got a few "roadmaps" on how to get there, but women are like states. Each one's different, and the map for Alabama won't neccessarily help me navigate through Kansas.
Of course, sometimes it's fun to get lost...and you usually discover new, wonderful routes.