If you mouse over the big number in the blue box, then the mouse tip title will tell you! Currently, it reads
7.5 questions per day on average over the past two weeks
From which we learn that it's averaged over the figures for the past 2 weeks. I think that's a rolling 14 day average, calculated roughly daily, but the very exact details probably don't matter too much!
According to a comment on this thread, it's a two week moving average, but I don't know where he gets his source.
edit we have similar stats available to us as mods, and our one is a one-week moving average...