lol.
I have the same 'problem'.
Last I checked its called Retarded Ejaculation. Apparently it is mostly psychological. Either you feel embarrassed by cumming to soon so you don't (and consequently find it hard to cum at all) or you may not be attracted/fully comfortable with the girl you are with.
For me personally, the first few times with a girl it takes over an hour for me to cum (and about 90% of the time I do that with my hand). But after getting used to her and becoming comfortable with her then I can cum in any amount of time. For example, sometimes I used to go home for quickies with my girlfriend. Depending on how late I was I could cum in either one minute, two minutes, or ten minutes.
The trick was that I was seriously horny. The other thing is, if you masturbate frequently it takes longer to orgasm between nuts.
Not that I'm advocating monogamy but if you have the same regular partner to practice on/with then the nut shortage will improve.