Per the manufacturer: The warmers all have a shelf life of at least 3 years. There is an "expiration date" on each warmer that is 3 years out. However, the warmers do not go bad, like a gallon of milk, once the expiration date is reached. 3 years is just the minimum time it usually takes for air to penetrate the outer film packaging and begin effecting the warmer. The longer after that date passes the shorter the heating duration will be. For instance if you have a warmer that is a year passed the expiration date it may last for 10 hours verses the 20. Two things to keep in mind though; The body warmers will produce heat for 20-24 hours. If a person uses the warmer for, say, 10 hours they can put the warmer in an air tight zip lock bag to cut off the air and stop the reaction...and the warmer will still have about another 14 hours of heating left, after it is opened again.