Some of them I never forget, I liked them so much that I remember almost everything (but it's also because I've re-read the ones I like the most so I'm less likely to forget... )
For most of them however, I remember the story fairly well, but I forget about some details, the names of the characters, etc. I couldn't say how long it takes me to forget, it depend how many books I've read since (details generally becomes a blur after I've read 5 or 6 other books)
What I forget most easily (I don't know why) are mysteries, I often forget the ending after a couple of months, I don't remember who's guilty, but if I re-read them it's very frustrating because it comes back to me in the course of the reading...
Usually however I have no trouble remembering books but I very quickly forget movies (sometimes I even forget having ever seen them until half an hour before the end)...