The key to keeping cereal fresh is to keep it in an airtight container. A tight seal keeps moisture and insects out of the cereal. Unopened cold cereal lasts for up to a year, but once it's opened, use it within two to three months. Hot cereal lasts up to six months.
After the Box Is Opened
Once it's opened, transfer the cereal to a container with a tight-fitting lid. Containers available commercially have snap-tight lids and come in a variety of sizes to fit your cupboard space. If you choose to use zip-top bags, close the bag partway, squeeze out the air, and finishing zipping the bag closed. You can keep the cereal in its original box by closing the inner bag with a bag clip, clothespin or even a bulldog clip.
Date and label the container or bag. As long as the cereal is sealed, there's no need to store it under special conditions, such as a cool, dry place. If your cereal is stale but not past the expiration date, try spreading it in a single layer on baking sheets and toasting it in a 400-degree Fahrenheit oven for a few minutes, or until it's crisped but not burned.