When serving corned beef you should put the leftover corned beef into the refrigerator right away after the meal.
Corned beef when refrigerated will last up to 4 days before it starts to turn bad and should be thrown out.
If you want to keep your corned beef longer you should freeze the corned beef instead of refrigerating it.
Corned beef will keep for up to 3 to 4 months in the freezer before going bad.