Mankey evolves into Primeape starting at level 28 in the main series Pokémon games.

In newer games where Annihilape exists, Primeape can then evolve further into Annihilape after it has used the move Rage Fist 20 times and then levels up once.

So in most games, if you’re just asking “when does Mankey evolve,” the key number to remember is level 28.

Information gathered from public forums or data available on the internet and portrayed here.