When I defeat a gym and get its prestige level to zero, is there a cool-down where only I, the conqueror, can place a Pokémon in it?
I wonder if someone could just wait at a public place for the moment when the gym becomes blank.
However, if such a cool-down exists, does anybody know how long it is?
From personal experience: I don’t think there’s a cooldown. I had beaten a gym some days ago and the game was pretty laggy. When I was finally able to place a pokemon, the gym was already another color.
No official information though, sorry.
EDIT: The game updated since I posted this answer. The answer is no longer correct! New answer is: It is not possible to steal someone’s gym now, battle winners have priority.