Summarize:

If you did not use all of the food in the packaging, put it into an airtight container immediately after opening it. Do not leave the food exposed to room temperature air for any amount of time. A plastic container with an airtight lid or a zip lock plastic bag is okay to use. Even if you have food leftover, do not use it once it has been left out. Throw the food away if it has been exposed to the air for 4 or more hours. After 4 hours, there is a chance of bacterial contamination. Put the airtight container into the refrigerator. Ideally, it should be kept at 40F (4C) in the refrigerator. Throw out the food if you have not used it within 5 days. It's also an option to store unused cat food in the freezer. First, check the packaging to see if there is anything warning against freezing the food. If not, divide the food into single-serve portions. This way, you can thaw individual meals as needed. Although food will last longer in the freezer, it is best to use the excess food within a month. Some wet cat food comes in cans, but you can also purchase refrigerated cat food. This type of food needs to be refrigerated as soon as you bring it home. If you have this type of cat food, once opened, it can be kept in the refrigerator for up to 5 days. If you haven't opened the food, you can keep it until the expiration date. Cats typically do not like to eat cold food. Add some warm water to the food. This will heat up the food but not make it too hot for your cat to eat. Don't mix your older, stored cat food with a newly opened container of wet cat food. Although both may be okay for your cat to eat, you may be unknowingly contaminating the new food. Serve the stored food first, and don't open the new food until you have used or thrown out the stored food.
Put the unused food in an airtight container. Discard opened food after 4 hours. Store the airtight container in the refrigerator for up to 5 days. Check to see if the excess food can be frozen. Keep opened refrigerated cat food for up to 5 days. Mix chilled food with warm water before serving. Do not mix new and old food.