Is there a point of diminishing return of fat being burned beyond 60 mins? Any max amount of time beyond 60 mins you recommend we should stop?

88 Minutes is the maximum point of time for optimizing your fat burn. After that, you run the risk of eating skeletal muscle which is highly valuable and you don't want to do that. If you have the commitment level to do 88 minutes, you will metabolize the maximum amount of fat without metabolizing any muscle.