I read 3 or 4 hours with the screen on at level 12 to 14 with WiFi constantly ON and my battery is down to approximately 70-80%.
If you are constantly running 3G due to WiFi issues, that is probably draining your battery pretty fast. You might want to activate Airplane mode.
Also, it takes a bit out of the battery to index books, even just a few like you downloaded.
I doubt we will see anything like the BS battery capacity that Amazon advertises. No way. But we should be able to at least read a few hours each day for several days.