There's really no set shelf life for brewing ingredients. Though some have Best By dates, the actual date they reach a point that they aren't useable, for whatever reason, depends largely on how they have been handled and stored. If you choose to use suggested dates or assign expiration dates, you can add them to the product.
You can edit the data for a product by selecting Ingredients/Ingredient type (hops, fermentables, yeast, etc.), and double clicking on the ingredient. Once you've done that, you can add to or alter the value(s) listed.