The active ingredient in PAM is permethrin. It's half-life is 30 days when mixed with water, and only 7 days if used outside and it's exposed to direct sunlight (many kennel and livestock pest control products have it). I don't know what its half-life is as the "PAM" mixture when still in the can.
I do have the Permethrin-10 livestock spray concentrate that I use to make mite spray, and there is no expiration date on the bottle.
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