The shelf life for the separate components of a liquid silicone kit is approximately 6 months. We say 6 months to be on the safe side, but we have had cleints who have used liquid silicone up to 10 years old and were still able to get it to cure! The base rubber will last for much longer, but the activator loses its strength over time and may have to be replaced to get the system to cure. Once a mold had been made, its “library life” depends on the type of catalyst that was used. Tin based systems will probably last about 5 years if kept in a clean storage area. Beyond that, it will become brittle and crack upon releasing the casting. If you suspect that a mold is very old, you can always use your last casting to make a new mold.
Platinum-catalysed silicones have a much longer library life, up to 20 years or longer.
The lifetime of a mold (how many castings you can get before you lose details or the mold begins to tear) really depends on what you are casting, how many castings you have made and if you use a release agent or not.