>>96508073
Level 11 is usually the best place to cut off. It simultaneously cuts off casters from getting 6th level spells, while also giving martials their extra 3rd attack. I also feel like in 3.5, the game got condensed and expanded from 2nd Edition. Being 20th level in 2nd is like being level 9 or 10 in 3.5.
Also, at CR 11 is where you start getting into the really crazy enemies, and it's always fun to fight them.