Depends on what's in the food and the temperature of your fridge (and even where the items are in your fridge - ie. the door doesn't stay as cold as the back). No longer than a week for most foods. I like to label foods when I open them so that I know how many days things have been sitting around. When in doubt, throw it out!

maybe a week or two. use your best judgment and don't eat something you're unsure about