After about 6 hours of reading with WiFi on and light level about 14 I'm down to approximately 2/3 charge (67%). Simple math suggests about 18 to 20 hours from full charge to a total discharge. Not very good if this rate of discharge holds up, especially since you should never discharge it below 30%. So it this rate I will get about 12 hours from full to 30%, so charging will be done quite often.
Next test I'll try using Airplane mode.