Regarding "on average"
What the articles linked by JohnDixon and me are missing is the fact that of that approximate dozen airworthy choppers per type a significant number are on missions in foreign countries like Mali or Kosovo.
Good luck for those who are not there in terms of family life.
Bad luck for their logbook.
And they are not exactly flying training sorties there either.
One could argue that the real situation is worse than the numbers suggest.