Does anyone use these? I have a couple and am wondering how long I should heat the wax. Am I supposed to heat and heat until the wax dissipates completely? I have one scent that seems to go on and on forever, while a different brand lasts for a much shorter but decent amount of time. The one that lasts a shorter time still has plenty of wax leftover, but no scent. I haven't gone to the end of the long lasting one yet. Just wondering if the short-lasting one is of poor quality or if the long-lasting one is of exceptional quality.