The idea that a spirit's grip weakens over time seems to be supported by Riley's recovery. Mia tried to drive the spirit away by recreating the seance and blowing out the candle, but it's not clear if that helped. It might have helped, but we can't be sure. Instead, we see a time jump at the end of the movie in which Riley's body is healed and he looks to be free of the spirit's grasp. The spirits may have greater strength over vulnerable people, whether that vulnerability is mental or physical.