You can store the finished cooked dulce de leche in the can for up to 6 months. Just make sure the can is fully dried to prevent rusting. Also, because you removed the label from the can prior to cooking you will want to write on the can with a permanent marker what is inside the can and the date that it is best used by (6 months from the date you make it). That way, you are not left guessing what is inside the can when you are looking in your pantry.