I just turned it back on and it's at 82% and was last fully charged 5 days ago. It's been on for 2hrs and 44min since then according to the battery screen.

It says it'll last until about 22:45 and that it's currently 05:10. (I apparently haven't set the time!)

I typically read for 30-60 minutes at night, I'd say, so those numbers make sense to me. Seems like about 20 hours.