Depends what I'm up to at the time.
When I'm in reading mode I can do a book a day, maybe more. (One summer, in my college days, I read 100 or so including the bulk of the ERB oevre back-to-back-to-back.)
On the other hand, when a new Elder Scrolls game comes out I can go months with only one or two.
I have a lot of alternate uses for my free time, these days, but in general I think I average fifty books a year, to be conservative.