Battery life figures might be interesting if you want to buy the unit. But if you are interested in designing in the MEMS display then energy to alter the display is key.
Clearly battery life depends on how many pages you turn.
Qualcomm don't give a battery life but say the reader should last for "weeks" based upon 30 minutes of daily reading time with WiFi turned off and with the front light set to 25 percent utilization.
That sounds like several hours of continuous use but going down if you are reading under the bedclothes with the front light on and the WiFi going.
Hope that helps.