Although I always carry a spare tube I also carry patch kits. Park tool ,1 glue-less, and 1 glue kit. They have been kept in my messenger bag and seen temps ranging from 90+ down to below freezing for the last year and a half. The kits are a rather cheap insurance policy for flats. But how long before its time to replace them? I'm guessing after a while the adhesive loses it effectiveness. What's a good rule of thumb?