The longest Boris has ever been able to hold a possession if 5 hours. It's incredibly taxing on him, and the longer he holds it the weaker he gets.
Also, it only works as long as the victim remains in the right mindset. It's easy when the victim feels scared and hopeless, so he tries to keep them feeling that way. If they manage to keep a positive, cheery attitude it is incredibly difficult for Boris to hold onto them.






