Room Temperature, Buttercream can be stored, refrigerated, in an airtight container for up to 2 weeks before using. Remove it from the refrigerator at least an hour before using, so it will warm to room temperature. It can also be frozen for extended storage. A decorated cake with buttercream frosting can be stored at room temperature for up to 3 days. If you want to refrigerate a decorated cake, place it in the refrigerator unwrapped until the frosting hardens slightly. It can then be loosely covered with plastic. Buttercream frosting can be frozen.