Some of the original kindles had ridiculous battery life without a backlight and turning off the wifi. Wifi and backlight kills most ebook readers.

I still have my kindle from a long time ago and it still gets around a week of battery life. I also have a kobo (because the kindle has some blotches on the screen) and it works wonderfully. It can work for about a week with heavy reading...but only if the backlight is mostly off and wifi is off.