Check it out - http://www.dailymail.co.uk/health/article-3522596/How-long-DOES-sex-normally-Study-finds-range-33-seconds-44-minutes-average-5-minutes.html

Turns out the median length of sexual intercourse (from penis entering vagina to ejaculation) is a little over five minutes. So, I'm feeling a little bit better about myself...

But here's an even more interesting evolutionary (and therefore relevant to this sub) tidbit near the end of the article:

Why we last so long is a pretty complicated question with no clear answer, but a clue may be in the way the penis is shaped.

In 2003, researchers showed – using artificial vaginas, artificial penises, and artificial sperm (corn syrup) – that the ridge around the head of the penis actually scoops out pre-existing syrup from the vagina.

What this suggests is that men's repeated thrusting might function to displace other men's semen before ejaculating, ensuring their own swimmers have a better chance of reaching the egg first.

Incidentally, this could explain why it becomes painful for a man to continue thrusting after ejaculating, since that would risk scooping out his own semen as well.

Never thought of that before. But now that I am...it would seem to make sense that the longer the penis, the deeper inside it goes, and therefore the more likely it is to be able to reach into every nook and cranny and pull out some other dude's load before it can fertilize the slut's egg. So, natural selection thereby picked bigger dicks.