Chemnitz:You know, the ones that come with cigar orders. Do they go bad or anything? Some of mine are getting to be over a year old. I mostly use them in my smaller humidors or zip lock bags.
But my rule of thumb that has worked fine so far is that the pillows are still good unless:
1) I notice any colored spots on the water pillow, 2) they start to feel "slimy" when refilling them, or 3) the water pools on top of the pillow and isn't absorbed when refilling.
If any of these 3 things happen I'll pitch them.