Heavy bag training, like other forms of exercise, stresses the body. Training too intensely may exceed the body's ability to recover. Only train a maximum of every other day. The more intensely you train, the more time off you should take between workouts.
In terms of intensity, remember that tendons and connective tissues need more time to develop than muscles. Therefore, the speed and power of strikes and kicks may develop faster than the body's structure is able to handle. Start with light to medium power and concentrate on proper body mechanics, and then gradually increase power. There is little benefit to continuously pounding the bag as hard as you can.
The length (time) of each workout is linked to the frequency and intensity. If one factor (frequency, intensity, or time) changes, the other factor may need to be changed to compensate.