I scan only about once every two weeks, or even once a month, without a particular schedule, and then it is usually a quick scan. About the same sort of frequency I'll scan with MalwareBytes, once again, a quick scan.
When Avast is first installed on a system I'll run a full scan with persistent caching, and I expect that's the last full scan I'm likely to do.
Over recent times,going back more than a year, it seems nothing is ever found. The scan with MBAM is mainly as a second opinion, to make sure the "no threat detected" message is indeed valid. (All scanners can miss the odd one now and then. Getting a second opinion reduces the odds of that.)
I'd only scan more than that if weird "symptoms" started to occur, that couldn't be explained by something else.