Generally speaking, Platinum Silicones have about a six month shelf life from the manufacturer. Now I've had silicone go bad in 2 months, and silicone last for two years. All you can do really do is mix up a small batch and test it.
As for Epoxy, I have never had epoxy go bad when kept in proper conditions.. room temp, no sunlight, etc. You may run into crystallization of the epoxy and the hardener will darken, but it should still cure fine.