I could be wrong and it was a long time ago, I can't even find the video anymore but I think it was a speech given by Dr. Machio Kaku, and yeah it seems mad! But that just because that's all us guys use it for ;)
I seem to remember hearing in a video seminar on the internet and technology in general that around 5% of the internet is porn.
From what I've read in the past anything under 3 minutes tends to be considered "premature" 3-5 I think is meant to be the average, but averages don't really mean anything, great sex that last 2 minutes is better than crappy sex that lasts for an hour xD