I just want to share this resource in case people didn't know it. It describes itself as the world’s largest open-source open-data library. And Includes Sci-Hub, Library Genesis, Z-Library, and more.
Anna's Archive is great. Finding this was a silver lining to the Zlib shutdown. I have it installed to the home screen on my android eink reader and its pretty seamless.
I couldn't tell you exactly because I generally plug it in when I'm done with it. It's not as long as my old Kobo Nia, which would last weeks. A 3 hour reading session it'll generally drop 10-15%, so I reckon you could probably get a week with daily use. That's with the backlight off but with wifi and bluetooth enabled, you could probably stretch it longer with airplane mode.
alternatively does anyone have any recommendations for such a device if OP doesn't respond? I've been missing e-ink since my kindle got destroyed several years ago