when does pidgey evolve

Pidgey evolves into Pidgeotto starting at level 18 in the mainline Pokémon games.

In most traditional games, that’s the first and only evolution trigger for Pidgey itself:

• Level 18 → Pidgeotto
• Then Pidgeotto evolves into Pidgeot at level 36.

If you meant Pokémon GO, Pidgey evolves into Pidgeotto by using 12 Pidgey Candy instead of a level, and then Pidgeotto evolves into Pidgeot for 50 Candy.