There is no specific level. There are many factors that dictate the level for each individual Pokémon. The factors are everything from the actual Pokémon, the moves you want it to learn (some moves can only be obtained by level up, but if you don't need/want any of those, you may as well evolve it the moment you get it), competitive or in-game, the list goes on. Generally though, the move set is the largest contributing factor, so once you have all the moves you need/want don't hesitate to 'stone' your Pokémon on the spot.
