Depends on all kinds of factors for me - and I really don't have an average time.
My girlfriend is a doctor who often has to work nights, and I have a pretty busy job myself, so there are times when we don't really get to see each other much for days. Naturally this means, more build up, and more desire. Usually makes it considerably faster, but oddly, not always. On occasion, the ol' soldier seems to have got so used to being upright, that he just decides he can stand at his post for as long as I like.
If we're being romantic, or having slow, gentle, loving sex, I can usually last considerably longer than if we're doing something kinky or going at it like bunnies on Viagra on the last day of the world.
Perhaps, most importantly, for me, are the reactions and behaviour of my gf. There are times when she seems so hot to me that she could blow on my crotch and make me cum. :bowdown: Okay, that's a bit of an exaggeration, but the point is that her her facial expressions, noises and general mood are easily the single biggest factor in how fast/slow, or strongly I get to the finish line. And she knows it, and takes full advantage of that fact. :1orglaugh
For me then, it depends on all kinds of things; mood, interval since last sexual encounter, what sex act we're performing, etc.