[HarryT]

The problem is, Katie, that you could only know how long the battery lasts if you were to read until you get the "Low Battery" warning, and who does that? I just recharge my Kindle pretty much at random every couple of weeks, so in all honesty I have no idea how long the battery lasts. Also, bear in mind that leaving WiFi turned on will roughly halve the battery life, so you need to take that into account, too. Another factor to consider is that the capacity of a lithium battery is highly dependent on temperature, so you'll need to know the temperature of the environment they're reading in, and understand the specific way that the battery discharge rate varies with temperature. Do you have that information available?

It's going to be extremely difficult for you to analyse any data that people give you in a way that will allow you to extract meaningful information from it.