My experience of the PRS-505 so far suggests that it will take a long time -- 2-3 years minimum, maybe more like 6-8 -- before the battery life drops enough that you need to recharge every couple of hours. LiON cells tend to degrade by 50% over about 24 months. The PRS-505 *seems* to have a lifespan of something in the range 12-24 hours (it may be longer -- it's hard to tell, it drains so slowly!) so after 4 years it *might* be down to 3-6 hours. But then again, it might still be up around the 12 hour mark.
And if you really don't want to buy a new reader in 4-5 years' time, you can always get a mini-USB charger that runs off AA cells and top it off every few hours. They're about a tenner on ebay. Right?