Borisb, kappert were absolutly right in the first post. If you not bootin android, and (if someone port it) boot for example in openink linux, with e-ink support only, it will work for weeks.
Android continuosly run some background jobs. Ok, trully any linux software running background jobs, or at least so called daemons (which monitors some events, like connections or key pressing)... BUT...
The e-ink optimised versions of linux (like openink) have something like zero jobs mode... and (what's more important) Marvell's processor (which used in EE) also have zero power consumption mode. What does it mean? You almost didn't need power for use EE in e-ink mode. The only time, when battery will be used is when new image rendering. So... let's say you need 30 seconds to read an e-ink page. Currently proccessor work in full mode while you reading (30sec) + when reader render next page to display it on e-ink (1sec). In zero power mode processor will work only when you press an button to display next page (1sec). It's 30times improvement in power consumption... Ok... there will be some battery drain to support wacom pen's input. But it's not important (as it use small amount of battery) and you also probably can turn it off.