6-10.
Low-1-5 - novice levels, by the end, your wizard gets it's iconic 3rd level spell slot and fighter his second attack
Mid 6-10 - journeyman levels, characters are competent and start to establish themselves as someone of notice, casters get 4th and 5th level spells, 2/4 archetype features kick in at these levels
High 11-16- experts, characters at these levels are powerful, they are somebody of notice in the world and they start to punch above their weight. Casters get their powerful spells of 6-8th level. Martials get 3 feats/asi-s and another attack
Epic 17-20 - characters of these levels are movers and shakers in the world. 9th level spells kick in. Martials can one shot decent amount of monsters with pure dpr.