An attic fan should run for at least 15 minutes at a time to ensure that the warm, moist air inside the attic is fully exchanged for cool, dry air. However, many products can be connected to a thermostat, allowing them to turn on whenever the heat inside the attic space reaches a set temperature. This set-it-and-forget-it method is a great option to ensure that you don't forget to manually turn the fan on.