Most beginners find that their max fat burning is between 70 and 80 percent of their maximum heart rate. (In comparison, seasoned athletes usually have a threshold between 85 and 90 percent of maximum heart rate.) As your fitness increases, you will move your max fat burning closer to your maximum heart rate. Over time you will also be able to sustain the intensity for a longer period.
For example, someone just beginning an exercise program may be able to hold his or her max fat-burning rate for 15 minutes, whereas a seasoned athlete will be able to hold his or hers for approximately 60 minutes.
To raise your max fat-burning rate you need to overload your cardiovascular system by working out once or twice a week around and above this intensity. It could be in the form of a tempo workout, time trial or long intervals, where you work out for five minutes then rest for two minutes and repeat the cycle several times.