I think with the Sony 505 you will finish reading your book before you need to recharge it. The e-ink screens don't use that much power and so in theory you should not need to recharge it often.
Actual life of the battery before the battery needs to be replaced? Not sure, it seems most lithum batteries last for 1.5 to 2 years of heavy use (think cell phones)? I don't know that I would consider a ereader as heavy use.
Don't mind Igor he is just a grumpy pants.