I have this "situation" as well, taking a very long time to cum. I haven't cum through fucking, blow jobs, or hand jobs for a while now. I'm sure it is due to stroking myself too hard, however in his response TopDudeFtl mentions holding off cumming, which is something I do. Stroke myself until I am close, and then backing off. Maybe that contributed.
I personally do not view this as a problem. True, the girls I hang out with become frustrated that they do not make me cum, feeling they are not holding up their end of the bargain. But on the other hand they do appreciate having a large dick to play with and fuck to their complete satisfaction.