The chance varies, depending on how many Pokémon you’ve defeated in the chain.
Chain length of 0 to 9: 0% chance of Hidden Ability
Chain length of 10 to 19: 5% chance of Hidden Ability
Chain length of 20 to 29: 10% chance of Hidden Ability
Chain length of 30 to 255: 15% chance of Hidden Ability
The length of the chain will roll over to 0 after it reaches 255, so if you have a chain longer than 255, the chance will go back down to 0%, then continue to go back up as normal.
Source: This reddit thread that also explains some other stuff, like IVs and the chance for a Shiny.
