They are dehydrated so they aren't like regular treats. I asked a vendor once and they said no expiration as far as they know of. I would assume unless they grow mold or something they are fine. There is no expiration date on the wrapper of the Merrick sticks that I sell. They never last long enough to worry about it
Personally I don't think I'd keep one more than two years... but again they don't last that long around here!