You can go up to 1 hours of cardio per day, assuming that you have the fitness level to perform interval training for that period of time ... Intensity is the key.

As long as yor protein intake is high enough (1.5 grams of protein per pound of your bodyweight per day ... divided evenly over your 6 daily meals/snacks/shakes) and frequent enough (meals 2 hours apart), you will keep yourself in a positive nitrogen balance and will minimize your muscle wasting.

Just like anything else, your body WILL adapt to a training stimulus if you do it the same all the time. And remember, to lose fat you need to create a caloric defecit. There are only a few ways to do this: cutting calories (a no-no) and increasing activity.

If you are still paranoid about losing muscle, take glutamine after your workouts and before bed. If you are STILL paranoid, take BCAA's (branched chain amino acids) after your workout.

Doses are dependant upon bodyweight ....