I wax my park skis (sintered bases) at least once a week (I try for twice, but sometimes I don't get around to it). I ski 5-6 days a week, so basically I wax my skis once every 5 times I ski.
Race skis, I'd wax 'em every time I need them to go fast...if I was just going to ski them around for the day I wouldn't worry about it, but for a race or practice, as close to when I need them as possible, no later than the night before, but if possible the day of the race, that way you know pretty much the exact temp and snow conditions and get get the best wax job possible.