Hours to a handful of days usually. If I knew where they were I would spend less time driving across the county trying to find places where the lights are still on where I can fill up my truck and gas generator. Since I got a briefcase sized gas generator I don't have to worry about food thawing or myself overheating in the summer since I can run box fans in the shade. They generally provide enough juice to power on a whole house natural gas heater in the wintertime.