how long do you regeat a casserole for
1 To cook a frozen family size casserole, add about 40 minutes to 1 hour to the unfrozen cooking time. 2  Deep dishes and very large size pans cooked from frozen may take up to 3 hours extra.3  The safest and best way to thaw frozen foods is to put them in the refrigerator the night before. To cook a frozen family size casserole, add about 40 minutes to 1 hour to the unfrozen cooking time. 2  Deep dishes and very large size pans cooked from frozen may take up to 3 hours extra.