If you really like the device and have decided to keep it for long time , then the moment you start seeing noticeably less battery life it's time to plan around procuring a spare battery and do not wait for it to be completely unusable.
I have a Kobo GLo HD that I bought when it was introduced. It's battery gradually degraded and has been unusable since the past 2 years. Now I'm finding it hard to source the battery. Indian retailers who had it are now saying out of stock and it's not exactly an obscure device.
Get spares beforehand instead.(if stored correctly, battery degradation due to being idle should be negligible).