@MissBoPeep: exactly — the food will tell you when you shouldn’t be eating it anymore. If it has the stank, toss it.
Sadly… that IS NOT TRUE !!
Food gone bad can indeed have NO APPEARANCE of being spoiled (taste – smell – or look)
A good guideline is up to 3 Days… 2 Days if you are dealing with Meat, Sauces, or Eggs.
And as others have said a lot also has to do with how you stored it (and how soon) after you prepared it initially. NOTHING should stand out for more than 2 Hours max.
For more info on Leftovers see this very informative link from the Mayo Clinic ( http://www.mayoclinic.com/health/food-safety/AN01095/ ) the text of which I have copied below…
== snip ==
You should eat refrigerated leftovers within four days to reduce the risk of food poisoning. If you don’t anticipate being able to eat leftovers within four days, freeze them immediately.
Food poisoning — also called foodborne illness — causes abdominal pain, diarrhea, nausea and vomiting. Food poisoning is caused by harmful organisms such as bacteria in contaminated food. Because bacteria typically don’t change the taste, smell or appearance of food, you can’t tell whether a food is dangerous to eat. So if you’re in doubt about a food’s safety, it’s best to throw it out.
Fortunately, most cases of food poisoning can be prevented with proper food handling. To practice food safety, quickly refrigerate perishable foods such as meat, poultry, fish, dairy and eggs — don’t let them sit more than two hours at typical room temperature or more than one hour at temperatures above 90 F (32 C). Uncooked foods, such as cold salads or sandwiches, also should be eaten or refrigerated promptly. Your goal is to minimize the time a food is in the “danger zone” — between 40 F (4 C) and 140 F (60 C) — when bacteria can quickly multiply.
Before eating leftovers, reheat them on the stove, in the oven or in the microwave until the internal temperature reaches 165 F (74 C). Because they may not get hot enough, slow cookers and chafing dishes aren’t recommended for reheating leftovers.
== end ==
Hope this helps,