The most effective heat therapy products are the ones that can maintain their heat at the proper temperature. "Warm" is the proper temperature. Patients should not have their heat source be hot to the point of burning the skin. The desired effect is for the heat to penetrate down into the muscles. Simply increasing the temperature of the skin will do little to decrease discomfort.
In many instances, the longer the heat is applied, the better. The duration that one needs to apply the heat, though, is based on the type of and/or magnitude of the injury. For very minor back tension, short amounts of heat therapy may be sufficient (such as 15 to 20 minutes). For more intense injuries, longer sessions of heat may be more beneficial (such as 30 minutes to 2 hours, or more).