Well, with my iPod, if I turn the background light off, and there is enough room, I can read from it for about 4-6 hours without charging. However, if you are near a plug in, you can have the iPod plugged in and still be reading. Otherwise, if you have the background light on while reading it will last about 2-2.5 hours. You can also change theintensity of the background light, and that will also make the battery life be longer.
All of this is for a 30 gig Video iPod, things may be slightly different with other iPods.
Kurt