I think that would be a difficult stat to gather. Actually, if one could determine the actual number of days or hours a member was logged on to MR, then calculate a posts per active hour, that might be more telling.
For example:
My post count = 6262
My join date = 08/02/2004
Total days = 703
Posts per day = 8.91
However, of those days, I probably only post, on average, 4 days a week, since I travel and seldom log on during the weekends. Also, on those days, I'm probably logged on about 5 hours a day.
Which means:
Active MR Hours = 2008 (a part-time job!)
Posts/AMRH = 3.12
So essentially, every hour I'm logged on to MR, I post 3 times.