You should store your epoxy resin and hardener at room temperature and keep in containers closed to prevent contamination. With proper storage, resin and hardeners should remain usable for many years. After extended storage, you can mix a small test batch to assure adequate curing, before using it on a large project.
Hardeners may darken with age, but physical properties are not affected by color. If clear finishing, be aware of a possible color shift if very old and new hardeners are used on the same project.
If opened, the epoxy has a 6-month shelf life if stored at room temperature in a closed box.