The duration is when fully rested. I'll make a cooldown for it.
Edit:It lasts for as long as he wants or until his energy runs out, which would not be a good idea because then he would be easy prey. As I said, his energy has to be sufficient to sustain it, and that takes a bit to regain his energy without doing anything, maybe a few turns. Remember, the time is contorted, so that may vary qute a bit.