Gym under attack. How long before it can resume?

I heard 6 minutes, but I'm not sure.

Was with a friend who has a second account that he uses to shave gyms. His other account took out the first slot of the gym. After there were only five slots, the Gym Under Attack thing came up.

He waited exactly 10 minutes and the text went away and he was able to finally put his pokemon in with his main account. So in his observation, it's 10 minutes.