Once you take it out of the wrapper, it starts to degrade. They are only good for about 4 hours. I have put them in and only used them for 20 minutes or so and then tried to put them in a tiny zip lock bag and it still did not work after I tried it again. If you do not see a little bit of smoke/vapor coming off of it and can smell the slight fragrance given off, its not working. Another good tell tale is that the skeeters are tearing you up. Dont take them out till you are ready to use them. Go ahead and stock up on refills. They might be a little pricey but they are worth it. Still might want to keep a bottle of skeeter spray in your vest for back up. If its pretty windy, the thermocell dont work that well and the spray will get you out of a bind.