Answer: 2 hours
There are important food safety guidelines to follow if your power is out. First of all, your fridge should always be set to less than 40 degrees Fahrenheit. I keep my fridge at 37°F. (If you don't know what temperature your refrigerator is, you can order a refrigerator thermometer on amazon. If the power has been out for less than 2 hours, your food in the fridge is fine (providing your fridge was set to the correct temperature in the first place). Some food that was held above 40° F for more than 2 hours should be thrown out. You will need to throw out the milk, meat, soft cheeses, and yogurt if they have been held above 40° F for 2 hours. You can keep the juice, jelly, ketchup, butter, and parmesan cheese longer. Here is a complete list of when and what to throw out. As soon as the power goes out, you can always fill a cooler with ice and put the milk and stuff in it in case the power doesn't come back on. That way, you'll at least have milk for cereal at breakfast. Any food that is surrounded by ice in a cooler, is staying cold enough to be safe. We had a terrible hail storm in the May of 2011 and lost power for 3 days. When the storm was over, my kids and I had fun collecting the hail to use as ice in the cooler.