I've only had mine for a little over a day. Yesterday I read quite a bit with and without the WiFi on and with the backlight at 25% or less. The battery didn't drain that much or very fast. Today I went outside and read. Even though I was in the shade, I had to turn the backlight up to 100%. The battery drained down by about 30%-40% of total charge in about two hours even with WiFi off. First, the backlight isn't bright enough reading in direct sunlight and marginal at 100% in the shade. Second, reading with the backlight at 100% is going to drain the battery within 4 to 5 hours, or so I estimate. But I probably won't use it outside much as I have a Sony PRS-950 that is perfect for brightly lit areas.

I think that under normal reading conditions for me with WiFi off and the backlight no more than 25% I should get at least 8 hours and probably a lot more.