I haven't noticed any kind of diminished battery life on my Kindle. And I use it a ton. Only when it's stuck indexing do I have the quick thought of "Oh, crap. This thing is dying!"
Then it gets out of the loop...and all is well. If/when the battery starts dying, I imagine it'll be when it's in the care of whomever I hand it down to once I get my mitts on my Paperwhite. And, then I'll probably just get the battery replacement thing that alan posted up above.