there isn’t an easy answer, as it’s always going to be dependent on the source you’re encoding from. That said, in my experience anything higher than CRF 25 on AV1 is noticeably compressed, even to a layman. I mostly work in 1080 and occasionally 2160, though.

i can’t remember the last project i worked on in native 720 (probably circa 2009?) and even now 720 is only part of my output pipeline if specifically requested by the client.