Leftovers will spoil eventually. Proper handling can extend the time that they are still fit to eat.
In the refrigerator a leftover will remain unspoiled for about 4 days.
The storage period of leftovers are affected by:
(1) whether or not the food holds contaminating micro-organisms
(2) whether or not the food is stored properly
(3) the tolerance of the eater
Potentially contaminated:
(1) raw/uncooked or poorly cooked
(2) out too long at room temperature or warmer
(3) beyond expiration date
(4) amount of preservative present (salt, acid, etc)
Stored improperly:
(1) temperature too high
(2) entered often
(3) improperly or inadequately sealed
The storage period can be increased to 2-3 months by freezing.
Factors affecting storage in the freezer:
(1) nature of the food
(2) any thawing and refreezing
(3) adequacy of the container
It may be desirable to completely reheat a leftover if it has been stored for a while.