How often do you read the books and how often do you watch the movies?
I'm guessing that you mean Harry Potter, and if you do, my answer is basically whenever I feel like it. I pretty rarely reread the books from beginning to end... usually, I'll remember a part in the books that I want to revisit and I'll start there. There are so many little details and moments and plots and stories within them that as soon as I realize I've forgotten exactly what happened, I like to remind myself with a reread.
I don't re-watch the movies unless I'm at someone's house and they're watching one. I don't own very many of them on DVD and I don't have cable television so I can't accidentally stumble upon an ABC Family Harry Potter weekend. I've always preferred the books so when I want to relive the stories, I reach for the books instead.