The ideal air exchange rate for a grow room is between 20 and 30 times per hour. This means that the entire volume of air in the room should be replaced by fresh air every 2 to 3 minutes. To calculate the air exchange rate, you need to know the volume of your grow room (length x width x height) and the capacity of your extractor fan (cubic feet per minute or CFM). Then, divide the volume by the capacity to get the number of minutes it takes to exchange the air. For example, if your grow room is 10 x 10 x 8 feet (800 cubic feet) and your extractor fan is 200 CFM, then your air exchange rate is 800 / 200 = 4 minutes.