Ereader

As with most portable devices, battery life depends heavily on what you do with the device and how you use it. CPU and GPU under full load are biggest power drains followed by display, especially the backlight but high refresh rate also requires energy. You probably won't be doing much CPU/GPU intensive tasks with an e-ink device (although you can watch videos and even play games on the Page if you are okay with the ghosting). E-ink displays doesn't require lights and they refreshes only when the content changes, so the display uses minimal amount of energy - unless you turn on the light. When you put the device to stand by it disconnects wireless connections and, I assume, it disables some of the background processes as well. Also it's not vanilla Android, but their own flavor of it. I don't know what changes they've made beside the user interface.

I've had the Page for couple months only, and my use cases have been all over the place, so I can't accurately comment on the battery life. It's several days with one charge, and I try to keep the charge level between 20% and 80% to prolong the battery, so I'm not even using the whole capacity. The battery life is not an issue.