A lot is the answer to your question. Well up here in the NE anyway. Fog rolling in conspired to cancel a flight last week. And Monday nights session was put off due to winds gusting 25-30 knots. On other ocassions I have also been cancelled once or twice for cloud base below minimums. I am in the fortunate position of having a decent job so can book in twice a week and "usually" get away with at least 1 flight a week which helps to keep my hand in.