I remember the last time I was asked this question I just quickly thought of the last 6-8 weeks of my training and gave a "50-60 or so" reply. (so similar to a poster above)
Upon a closer examination of my running log from 2009. The results below:
MPW: 42.2 mpw. (standard deviation of 20 mpw)
Range: 0 to 74 mpw. (4 weeks of 0, and 6 weeks of 0-20 which were probably beginning/end of training blocks)
Average excluding those 10 weeks below 20 mpw = 50.5
Average for the top 26 weeks = 57.7.
So I guess if I were honest I should've said 40-60, although 50-60 during weeks when I am "training" would be technically correct.