I haven't found any since June in either of my colonies, either on the board under the OMF or in drone brood. I doubt that they're completely mite-free though. The little monsters can get in by a variety of means, and build up steathily from small patches of infected brood that can easily be missed.
Ungainly-looking adult mites can hop onto a passing bee with amazing speed and accuracy. Even if they are absent right now, you can't afford to stop checking, because it's the thievin' season and if your bees are drifters, they can still possibly pick up mites from another colony. Likewise, any scurvy robbers that manage to get past (or just jostle up against) your guard bees can also introduce new mites to your colony.
Not that I'm complaining about not finding any!