When does your stomach burn fat and when does it burn muscle?

Your stomach doesn't burn fat. Your body does.

If you mean when does your body burn fat, it's simple. When there is cortisol in your system from a stressful workout and your muscle mass is being used and maintained, you will burn fat. If cortisol is released but your muscles aren't being used much as in steady state cardio, muscle will be burned.

Focus on short, intense workouts and use your muscles rather than steady, boring cardio. This will make you fat and weak.