I wouldn't say that it hurts for hours, more like ten to thirty minutes. It also isn't the best move because the second a male sees that leg lifting, he will assume that his balls are the primary target (the
telegraph Chattering Magpie referred to). By the time you have executed the move, he'll probably have
formulated at least one block.
NEVER attempt this move on a male who has sisters. It's just about the dumbest thing you can do! He has probably taken so many shots to the crotch that 1) it just doesn't hurt any more (much like being repeatedly maced), and 2) he probably has about nine counterattacks that he's perfected to the level of Bruce Lee. In fact, if there was a martial art that consisted entirely of ballkickery avoidance techniques, he'd probably be the Senior Grandmaster.
Physiologically, it's difficult to get this right. The penis is in the way. If your foot lands on the dick it just isn't going to hurt at all, because it isn't exactly tuned for pain reception. There's a delay of at least ten seconds before it really kicks in. It has a very long, shallow attack, peaks as a dull ache that seems to last for ages, and has a fairly long release. See also blue balls, ADSR. And unless you are accustomed to close-range combat, you may screw up and land short.
Psychologically, as has been noted above, it is a very ill-advised move. As males, we grow up knowing about the ballkick from a very early age. We know that you girls know what it does to us, and we know that we're not supposed to hit you back. The state of affairs is this: If a woman hits a man without reason, he wishes to counterattack - an irresistible force. However, cultural influences encourage men to refrain from counterattacking, even if the woman attacked out of sheer spite - this is the immovable object.
The paradox breaker is the ballkick. It is considered such a spiteful, outrageous move, that the immovable object suddenly disappears, leaving nothing but rage. This amps the shit out of the adrenaline gland, which can delay the painful effects of ballkickery for up to several minutes (as confirmed by an acquaintance of mine who's been in fights too numerous to count). Unless the guy is spineless or very inexperienced with pain in general, you've just released all his inhibitions and granted him free license to tear you a new asshole.