Ok... I voted Never. There are just too many books in the world that I don't want to spend time reading the same one twice.
That said, I do save my books and others in my house, mostly my teen soon read them too. He has read many of them more than once. He read the full Sword of Truth series 3 times I think.
BTW: How do you folks keep track of book's you've read. I was thinking of getting a Groqit (
http://www.groqit.com/) but it won't work well with eBooks.
BOb