Leftovers can be kept in the refrigerator for 3 to 4 days or frozen for 3 to 4 months

Bacteria grow rapidly between the temperatures of 4° C and 60° C(the danger zone). To ensure that leftovers are safe to eat, immediately after cooking refrigerate or freeze the leftovers for rapid cooling. Throw away all perishable foods that have been left in room temperature for more than 2 hours (1 hour if the temperature is over 32° C).

Cover leftovers, wrap them in airtight packaging, or seal them in storage containers. These practices help keep bacteria out, retain moisture, and prevent leftovers from picking up odors from other food in the refrigerator.

To prevent bacterial growth, it's important to cool food rapidly so it reaches as fast as possible the safe refrigerator-storage temperature of 4° C or below. To do this, divide large amounts of food into shallow containers.

Leftovers can be kept in the refrigerator for 3 to 4 days or frozen for 3 to 4 months.

Reheat

Safe ways to thaw leftovers include the refrigerator, cold water and the microwave oven. Refrigerator thawing takes the longest but the leftovers stay safe the entire time. After thawing, the food should be used within 3 to 4 days or can be refrozen.

If a large container of leftovers was frozen and only a portion of it is needed, it is safe to thaw the leftovers in the refrigerator, remove the needed portion and refreeze the remainder of the thawed leftovers without reheating it.

It is safe to reheat frozen leftovers without thawing, either in a saucepan or microwave (in the case of a soup or stew) or in the oven or microwave (for example, casseroles and combination meals). If using a microwave, be sure the covering is microwave safe, and vent the lid or wrap to let the steam escape. Reheating will take longer than if the food is thawed first, but it is safe to do when time is short.

When reheating leftovers, be sure they reach 74° C as measured with a food thermometer. Reheat sauces, soups and gravies by bringing them to a rolling boil. Cover leftovers to reheat. This retains moisture and ensures that food will heat all the way through.

Sometimes there are leftover "leftovers." It is safe to refreeze any food remaining after reheating previously frozen leftovers to the safe temperature of 74° C as measured with a food thermometer.

More info at https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/leftovers-and-food-safety