It would totally depend on how much forplay was done... with foreplay I can't imagine going more than an hour... no foreplay a couple hours...?

You also have to be sensitive to the likelyhood of your girlfriend drying up, or if she's actually getting closer to orgasm or not. Otherwise it sounds like alot of hard work for nothing... and coming from a chap, I think it shows alot of understanding for the fact that sex is about the two of you. If one of you is not on the same parallel, then you've got to respond and be symapathetic. I think there is too much focus on the end result sometimes and even the big ego trip of lasting until the cows come home. For me, the whole cycle of love making, the round trip if you follow, is what I find most satisfying. It can just as well be a walk round the block or a day trip up a hill.
But hey, if it's going well, the juices are flowing and your're not just trying to overcome muscle fatigue, the sky's the limit superman!