Shelf Life: The shelf life of an open container of hardener or resin is about 6 months to 1 year. Some things that may accelerate the yellowing process of the hardener are exposure to oxygen, improper storage, such as being kept in a cold environment or the product coming in contact with moisture. Keep in mind that hardeners may darken with age, but physical properties are not usually affected by color. You may still be able to use your epoxy without an issue. I recommend that you mix up a batch and test a small area before you attempt to do a large project to be certain that your cure is still crystal clear.
Storage: You should store your epoxy resin and hardener at room temperature and keep in containers closed to prevent contamination.