I use potable aqua, and even though they have a 3 year shelf life, thats only if you don't open the bottle. Then it's six months. So, what I do is every six months, dispose of the ones in my PSK repacks, replace them with the ones from my ditch kit with the set I have on hand for when I assemble BOB, and put new ones in the standby box (BOB's home when he isn't wearing his skin).

For everyone else I build kits for, I recommend an annual replacement, just becuase that and firestarters are the one place I'd rather dispose of perfectly good gear than risk having something stale in my pack.