I have purchased the medium size warmers to replace old ones in cat beds I have purchased that have outlasted the warmers. I have never had one of these quit on me. I replace them when they start overheating which eventually always happens when they get older. My cats like heat most of the year, so I cannot complain about how long they last. They are the best quality I can find, and while the price may seem a little high, I think it is very reasonable for this type of product.