The problem is defining "use time". With eInk readers, the only time it needs to be actually awake is when refreshing the screen, and then it goes back to sleep immediately after. So depending on your reading speed, one hour of active reading time - which is what the users themselves would normally think as "use time" - can vary a lot in how much power actually gets used.
That said, I do think they should just give the number in terms of page refreshes and let the users figure it out from there.
That said, I do think they should just give the number in terms of page refreshes and let the users figure it out from there.