I have a list of go-to books based on what I want out of the experience; for example if I'm down and out I go to Mercedes Lackey for her 500 Kingdoms and fairy tale re-tellings because they're silly and ridiculous (to me). My criteria for re-reading books is based on the question "Are the details of the story fuzzy to me or have I completely forgotten huge swaths of the story? Yes? It's time to read it again!" This usually takes about six months per book.
And! If I've checked a book out more than three times from the library I add it to the to-buy list and keep an eye out for it at used bookstores, yard sales, flea markets and friends of the library sales for somewhere between 6 months and a year. (Each book on the list is dated when I put it on there.) If I can't find it within that time period I will buy it new from a bookstore or online if necessary.
I actually re-read books so often that I have to make a point of adding new authors to my regulars.
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And! If I've checked a book out more than three times from the library I add it to the to-buy list and keep an eye out for it at used bookstores, yard sales, flea markets and friends of the library sales for somewhere between 6 months and a year. (Each book on the list is dated when I put it on there.) If I can't find it within that time period I will buy it new from a bookstore or online if necessary.
I actually re-read books so often that I have to make a point of adding new authors to my regulars.
As a side note, I rarely re-read fanfic.