Question: how long do you have to wait to put a pokemon in a gym?
Answer: If you get the "The Gym is under attack! Try again later" error in Pokémon Go you typically have to wait 10 minutes before you can add your Pokémon to the Gym. The thing is, though, you don't actually have to wait. You can fool Pokémon Go into thinking you've waiting simply by setting your clock forward 10 minutes.