Generally speaking, our silicones have a very long shelf life. We've used as well our customers have informed us they have used our silicones very effectively after sitting for years. The official shelf life of all Alumilite's silicones is one year from the date of purchase in unopened containers. Moisture will react with some of the liquid silicone bases and cause them to thicken and go bad which is why it is important to store in ambient temperatures in a dry environment. Replace caps after using and avoid storing in garages or other non-temperature controlled environments. The mold-making putties are very stable but will tend to slow down over a year from manufacturing. They still work but the open time and de-mold time will begin to extend. If it sits too long, the catalyst in the putty can lose it's reactivity (not allow the chemical reactions to do their job) and could lose its ability to cure properly.