Your fitness gains depend on the frequency, intensity and duration of the aerobic exercise. Each session should last from 20 to 60 minutes and, if you can’t exercise daily, work out at least 3 to 5 days per week at an intensity level measured by heart rate 60% – 90%. According to the American College of Sports Medicine (ACSM), during the first 15 minutes of aerobic activity, glycogen or sugar within the muscles is used for energy. Fat metabolism for energy takes place 15 to 20 minutes after you begin aerobic activity. This is why it’s important that aerobic duration be at least 30 minutes. However, aerobic sessions longer than an hour continue to burn fat but at not the same rate as during the first hour.