You dont 'start burning fat' at any point. You will always burn C, P and F it is just that certain factors will make you burn more of one and less of another.
Basically, if you are unfit and you run for 30 mins you will struggle to breathe and therefore wont be able to burn fat efficiently bcos fat metabolism needs oxygen present. So as you get more and more out of breath you will use glycogen (carbs) more and more and less fat will be broken down.
However, if you are very fit, you might be able to go at the same pace for 30 mins without even gasping for breath so you will burn a lot more fat bcos oxygen is present.
Forget 'fat burning zones'- it is nonsense. Eat well, do cardio, do weights and be active- the only way to burn fat.