I was looking when I posted the comment, but can't find the relevant studies.

This is the best I'm seeing atm. https://www.medicinenet.com/how_long_do_you_need_to_fast_for...

Most of the studies we see in IF and autophagy are mouse studies. It isn't to say fasting doesn't result in autophagy in humans, just that the time window most IF practitioners use is not long enough.