I have never charged my DX, never turned it off
I connect it every day (or sometimes every other day) to USB for a new online news from calibre - 10-15 minutes. That must be enough to recharge what I have used from the baterry when reading or occasionally truning on 3G for browsing web. I have once spend long afternoon browsing web, about 4h, and then the battery dropped significantly leaving about a quarter.