How much do you read a day? My Kindle keyboard gets between 2-3 weeks on a charge. Since it is 2.5 years old, I have seen some degradation but nothing like you are claiming.
The issue is if you don't use it during large stretches of time, the battery will not charge correctly again. I'll use it a lot for a month and then I might not read anything on it for a couple of months. It's a documented issue with the Kindle keyboard (the other e-ink Kindle readers don't appear to have the issue). Since it depends on the Kindle type you are using, I think it's either the battery used or a design issue with the battery charging circuitry in the Kindle Keyboard.
If you use it often and keep it charged, it shouldn't have a problem. If you don't use it for awhile and the battery runs down, you will probably have issues from that point on.