The leftover casserole is usually good for up to 5 days in the refrigerator as long as you keep it refrigerated.
So as long as you eat the leftover casserole within 5 days and keep it refrigerated for those 5 days until you eat the rest of the leftover casserole then it will still be good and safe to eat.
If you want to keep the leftover casserole longer than 5 days you should freeze the casserole.
When you keep the casserole frozen it will last up to 6 months to a year before it loses it's freshness.
The casserole will still be safe to eat after a year however it's best to eat it well before a year even when frozen because it won't be as fresh or fresh tasting.
But the casserole as long as it's been kept covered and frozen it will still be good to eat a year later.