I have bought kindle books since 2008. As far as I can tell, I can still load books I buy today on my K1 I got in 2008. Although its a bit slow with calling home at this point. I charge it about every month or so to keep it alive and like to try it out by putting a book on it. Just to see the shiny sparkly bar.
So at this point my library is backed up from today back to 2008.
If something changes, I'll figure it out going forward.
Reminds me I need to get another external storage as mine went kaputt. But since my books are on my computer with calibre and also in a cloud, not all is lost.
I already lost all my paper books once when moving from one continent to another. That was painful. Its a bit easier to take care of an ebook library.