To start, set the heating pad on the lowest setting. For minor aches and pain, a low setting might be more than enough to reduce pain and stiffness. You can gradually increase the intensity of heat, if needed.
There are no hard or fast rules regarding how long to use a heating pad on your back. It all depends on the level of pain and your tolerance to heat. Even so, if you use a heating pad on a high setting, remove after 30 to 60 minutes to avoid burns.
Be aware, though, that putting a heating pad on a new injury (generally within 72 hours) can worsen pain-causing inflammation. Frequent, prolonged, or otherwise improper use of heating pads has been associated with burns and other ailments. If in doubt, consult a doctor before treating yourself with heat.