It's hard to tell 'cause I read and reread them to my daughter (don't you love being read to- I swear it's the nicest thing in the world!!) and, of course read and reread them myself. Then there's the having to read bits to answer questions and so you end up reading on from the part of the story you jumped in on.. but I estimate that reading the books cover to cover:
Philosopher's Stone - 7 times
Chamber of Secrets - 4-5 times
Prisoner of Azkaban - 4 times
Goblet of Fire - 8- 10 times (my fav.!)
Order of the Pheonix - 3-4 times
HBP- I WISH!