Treat leftover blueberry dump cake just as you would a blueberry pie. It can be left out for up to two days. Make sure the pan is covered with plastic wrap once the dessert has been cut. If you will need to store it longer keep it in the refrigerator, preferably in an airtight container but covered in plastic wrap will do. It will be good there for 5 days if you put it away as soon as it is cooled to room temperature.