I know cardio at 65% on an empty stomach is supposed to burn fat off but how long can you do it in one session for before the law of diminishing returns sets in. I do my for 45 min which is really 30 mins because it takes 5 to get to my heart rate and then I cool off for another 5. I Would do it for 2 hours at a time but I guess that will start eating muscle. I am doing it 3 times a week now but Mon I am starting the Iron Man HIT so I can do cardio 4 days a week. If I could go longer than 45 mins that would be great but I want to keep as much muscle as possible.