25%, but if the Pokémon’s HP is not divisible by 4, it rounds down. The substitute’s HP is whatever was taken from the Pokémon exactly.
By decreasing its HP by 25% (rounded down) of its maximum HP, the user creates a substitute with the same amount of HP as it lost.
Ex. 1: Mr. Mime has 400 HP. If Mr. Mime creates a substitute, then Mr. Mime will have 300 HP, and be protected by a 100 HP substitute.
Ex. 2: Mr. Mime has 399 HP. If Mr. Mime creates a substitute, then Mr. Mime will have 300 HP, and be protected by a 99 HP substitute.
Hope I helped!
Substitute Bulbapedia
