Q: how long can I expect the battery to last when listening to audio books

May 30 hour with audio book since that is the same as music.

Radio is a lot less since that requires wifi. Maybe 10 hours

What about listening to audiobooks on YouTube, would that take as much or more battery than radio?

If you use YouTube from Safari, it will consume more battery since you can't turn off the display or the audio will stop. I don't know if the YouTube app is capable of that.

Thank you for your prompt reply