Very recently i had actual sex for about 2 hours non stop..... one time (last summer) i went for 4 hours straight (it was the first time with that chick as well, lol... poor thing).... sometimes just half an hour.... but i've ALWAYS been told i go for a really long time apparently. I've never watched other people have sex, so i don't know what average is.
About 15 years ago i used to purposely let my mind wander (during sex) when i got too aroused, then bring my mind back to the sex again when i wasn't so aroused... to last as long as possible.... but i think over the years that just became automatic or something, cos i don't need to conciously do it any more.