I estimate that I got about 12 hours going from a full charge to about 30%. I kept WiFi on and the light level was about 14 most of the time. Since it is not a good idea to allow Lithium Ion and Lithium Polymer batteries to drain too low, then I'm looking at recharging every 2 or 3 days.
Amazon claims: "A single charge lasts up to eight weeks, based on a half hour of reading per day with wireless off and the light setting at 10. Battery life will vary based on light and wireless usage."
That translates into 28 hours with no WiFi and light level at 10, so I guess 12 hours with WiFi and light level at 14 is about what should be expected. Still...that is a lot of recharging and that means a shorter life for the battery.