I run fans 24 hours in my closet. It doesnt take long, even with the door open, for plants in the back corners to show signs of stress if I turn it off- and temps never climb above 80 in there. It's simply suffocation. In flower, I also run 24h fans but the bulk of the exhaust comes on at lights-on. Remember, it's not simply the heat you have to worry about, it's fresh air exchange, unless you run CO2.