I got some 20 hours out of my M1 Air when I tested it after the first full charge. Then I decided to charge it. Calculated at various points that it would last roughly 25 hours and it sure seemed like it was going to.

Much of this time I had Xcode running and videos playing, etc.

Subsequent charges never lasted this long because I installed more bloat, but still always over 10 hours even when I had a bunch of shit running.