well thats like saying how long is a piece of string, it depends on many factors i.e. your position in the food chain, how busy the school is, how long are the opening hours of the airfield, and what the weather is like.
my own experience was that shortly after the millenium, I did as much as 75 hours a month in june, july august, which could fall to 15 in december, january.
but i suspect peoples personal experiences vary greatly.
regards
CF