You can keep hash brown casserole in the fridge for up to 3 to 4 days before it goes bad.
If you want to keep the hash brown casserole longer than 4 days you should freeze it and when frozen the hash brown casserole will keep up to 6 months and sometimes longer.
However after 6 months the hash brown casserole will not be as fresh as it was before but it can still be okay to eat even though it might not look very appetizing.
When freezing or refrigerating the hash brown casserole be sure to cover it tightly to keep other food smells from getting into the hash brown casserole.
I would recommend eating the hash brown casserole within 6 months when you freeze it to have the freshest taste.
If you're just refrigerating the hash brown casserole then eat it within 3 days so that it doesn't go bad.
You can just reheat the hash brown casserole in oven for around 5 or 10 minutes or reheat the casserole in the microwave.