It takes all of a minute, but even to him, it feels like much longer. It's only once he knows that pushing any further would leave Jack permanently scarred that Castiel starts to draw himself back, forcing his Grace back in to the confines of his vessel, and his hand slowly- carefully -slipping free.

With the connection broken, he stumbles backwards, his grip dropping as he puts some distance between them. The rush is still there, his eyes wide and hands shaking. So he takes his time, leaving Ianto to deal with Jack for the time being. He needs to get himself back under control again.