A quick jaunt to Wikipedia, that font of semi-accurate information, reveals that some men do not emit precum (aka Cowper's Fluid) at all. I pity those lads. Not that I'm a complete manwhore, but I have been with more than a few men and don't ever remember one who didn't emit at least some precum.
On the other end of the spectrum, some men emit as much as a teaspoon full during a sexual encounter. I'm one of those. When I was young, my oozing willy constantly embarrassed me. I'm reminded of the scene from All that Jazz where the young protagonist gets up on stage in white pants and is mortified when the audience laughs at his visible precum stain. That never happened to me, but when I saw the movie, I knew exactly how the lad felt.