Most manufacturers of epoxy will list their shelf life for their products as 1 year for unopened epoxy.  This of course depends on the formulation and storage conditions.  These recommendations are for use of the product to obtain it's designed specifications upon application.  In our case we are not dealing with extreme specs. for modelling use.  I've used epoxy that is several years old with no ill effects.  Storage is the key.  Keep your containers closed when not in use and store at normal room temperature out of direct sunlight.  I'd say for our usage, epoxy up to 5 years old can be used and will give satisfactory results. 

I have used some 6-minute epoxy that was over ten years old and it worked fine. It was stored tightly capped in a closed cardboard box in a back room of the house.   It had been opened before.  Unopened should last even longer.  If it will still pour, it should work OK, IMO.