If max fat-burning is the goal, I think 15-20 minutes of HIIT is optimal. But it's clearly more fatiguing than low intensity. I think low intensity is more likely to be muscle wasting when the duration is long as people do to try to compensate for it burning less calories and revving the metabolism less than HIIT. But I don't view shorter low intensity sessions like Waterbury recommends as catabolic. They can help recovery and a good way to burn some extra calories if that is the goal without taxing the nervous system.