That depends on the purpose of fitness. First of all, we should know the "maximum heart rate" (the prevailing algorithm: 220- age). Generally speaking, for the sake of health, 60% maximum heart rate is recommended. "Fat reduction", 60-80% of the maximum heart rate. Muscle exercise, HIIT or other extreme training, the highest heart rate is over 90%. Obviously, the closer the heart rate is to the maximum, the shorter it can last, and the coordination of exercise time and intensity is the key.