I also suspect the battery life on my new PRS700 isn't up to snuff (or else the specs arn't). When I use the light I have it set to the lowest setting. There is no way it lasts 8 hours. I'm too lazy and busy reading to keep a log of the times I'm reading with and without the light (a future project maybe, after I enjoy a few more books). I seem to be charging my battery about every other day, and I don't read 4 hours a day.
It is not a hassle for me to charge it frequently, but still...