What to do if i have muscle already but i'm a little bit fat, so i know if the cardio is over done it can burn my muscle, so how long before it gets to that point?

Eat enough: If your cardio is balanced with a healthy diet, you,should slowly lose any fat that you are carrying. You will not be likely to lose muscle unless you are very underweight or restricting food intake excessively. You may want to add some light lifting to balance your routine. The main point is to remain moderate in diet and to accurately asses your level of "fat."