I know I was quite surprised at the stack of
4e books that a friend of mine had only 10 months after release. It was nearly as tall as my 1e hardback collection after 10 years of that edition.
Sooner or later, the potential content of books is going to start to peter out (maybe it'll get repetitive, maybe overspecialized), the sales of individual titles will slip, and they're still going to need to fund their business model. That next edition is going to look pretty attractive. My guess is we won't go until 2015 to see it.