Rice Krispies need to be stored in an airtight container or jar at room temperature. Even under these conditions, Rice Krispies will not last you more than 2-3 days.
To ensure they last longer, freeze them for up to 6 weeks. Then, store them in layers separated by wax paper in airtight containers. To use them, remove them from the refrigerator and keep them at room temperature for fifteen minutes before serving.