I believe they do not last long before they fail. You would be better of with a uth connected to a thermostat to regulate the temperature.
http://ball-pythons.net/forums/showthread.php?t=201740
Visit this thread for more info on glass tank setup. You want to know how much heat is being produced and a bp needs heat for 6-24 hours i believe to aid in digestion after eating. Hot rocks are not the best way to do this.