You spent your whole Sunday night cooking extra food for the week. Or maybe you're single and you followed one of those recipes that's supposed to feed four, because nobody ever posts recipes with single portions for some reason. Days later, you find yourself eyeing the Tupperware in your refrigerator, trying to figure out if it's ok to eat and relying on the smell test. If this is you every week, you might want to read on.
In order to make sure your food is safe to eat, all you need to do is keep track of when you cooked the food and do a little simple math. The USDA has a pretty standard rule of thumb: if it's been sitting in your fridge for more than 4 days, chuck it. It might not smell like rotten eggs or look like a science project, but it might not be safe to eat. Use your best judgment, but try to use your leftovers within those four days. If you have a bunch of stuff on day four, it might be time to throw everything in a stir-fry and finish it that night.
If you don't think you're going to eat your leftovers by the fourth day, consider sticking them in the freezer. The USDA says you can keep food in there pretty much as long as you want, as long as it doesn't thaw. While it will be safe pretty much until the apocalypse, it's best to use your frozen food within four months in order to get the maximum flavor and texture out of it. You can save it longer, but it might get freezer burn.
The best way to freeze your food is to place it in shallow containers or gallon sized freezer bags with the food flattened out. It will freeze faster this way, which not only preserves taste but also keeps bacteria out.
When you're ready to reheat it, bring the temperature of the food up to 165 degrees Fahrenheit to keep any lingering bacteria, and chow down.