Once or twice a month. I take it all off when the polish starts to chip or fade and go with bare nails for a week or two until I paint them again. The way I see it, you're either wearing polish or you're not - there is really no excuse to have chipped half-on, half-off polish, and it looks tacky :P
Weeelllll, don't get me started on my nails, I paint them all the time for the appropriate occasion, if there isn't an occasion, pink all the way ;) ahaha I do it every now and again when I feel like it ;)
I'm not a nail polish kind of gal. I used to wear black nail polish 2 or 3 years ago, but that was just a phase. I don't really wear them partially because I end up scratching off all the polish out of bad habit.