I reckon that until battery technology changes, we have this:
-Light to moderate usage, coupled with original, efficient iOS versions: good battery life.
-Moderate use, original iOS version: decent battery life
-Heavy usage, original iOS version: mediocre to poor battery life.
-Moderate or heavy usage, updated enough: atrocious battery life.
If M processors haven’t precluded this from happening, I wonder whether it’s possible at all, until battery technology changes. So, use it lightly or moderately, battery life will be good. Crank the brightness up, and it won’t be. Just like every iPad since the 1st-gen. No processor improvement has been able to give good battery life with heavy use.
Like I said earlier, I noticed a massive difference on my M1 iPad Air (5th-gen) with very light use. Battery life on M1 Macs is widely considered impressive, and the iPad does not fall short. But you need to be a light user.
I haven’t tried a 12.9-inch M-processor iPad on its original iOS version, it would be interesting. Neither have I tried an older one, like a 1st-gen iPad Pro on iOS 9. If it’s anything like my 9.7-inch iPad Pro or a 6th-gen iPad, expect 14 hours with light use. For an M-processor one, expect 25. But I don’t know whether the screen overpowers the battery even with light use.