From a food safety standpoint, the answer is not very long at all. Most leftovers already have one strike against them when a diner asks for a doggie bag or a home cook cleans up after dinner. The food itself has had time to cool down to a temperature range that encourages bacterial growth. Many food safety experts say that prepared foods can only survive for two hours at room temperature before they become too contaminated for safe ingestion. Only immediate refrigeration or reheating can render leftovers relatively harmless.

Opinions on how long properly stored leftovers will still be edible vary from source to source. Many experts suggest a maximum of two days for all types of leftovers, beginning with the initial refrigeration. Others suggest that leftovers containing meat should be eaten within one to three days, while other leftovers may last as long as three to five days. Meat products tend to attract more serious food-contaminating bacteria such as salmonella or E. coli. Vegetables not prepared with meat may stay edible a little longer.

It is important to store leftovers properly in order to avoid complications later. Use tape or a wax pencil to mark the dates the leftovers are added to the refrigerator. Do not store the leftovers in their original cookware to avoid contamination. Divide large amounts of leftovers into smaller containers to help keep them cool. Keep raw meats on lower shelves to avoid cross-contamination by way of dripping juices.

If you don't know the history of leftovers, it may be best to discard them immediately. Foods brought to a family picnic or other outdoor event may have already exceeded their two-hour safety margin. Some leftovers become too contaminated to eat in an hour or less, especially during hot weather. Foods prepared with mayonnaise or other egg-based products may not be safe to eat as leftovers the next day, despite efforts to keep them refrigerated.

Some people may still feel the need to consume leftovers after a few days have elapsed. As long as the food is reheated sufficiently and does not have an off flavor or odor, a healthy adult may not experience symptoms of food poisoning. The human digestive system can handle a certain amount of food contamination without major complications. Leftovers should be discarded at the first sign of spoilage or discoloration. Those mold-covered 'science experiments' in the back of the refrigerator should also be discarded to prevent cross-contamination.

In general, set an amount of time you feel comfortable storing leftovers and stick with it. After three days, for example, pitch every carry-out box or foil-wrapped mystery meat in your refrigerator. Consider ordering smaller portions when dining out, and avoid the temptation to stock up on leftovers from a picnic or wedding reception. Only take the amount of leftovers you can consume in a few days or store in a freezer. Remember, the two hour safety zone begins at the time of preparation, not when you arrive home after the event.