It depends. If you have another fan bringing fresh air into the room with the correct CFM rate then you will be fine. If you need your “grow light” fan to keep the air movement optimal, then I suggest running it all of the time. HERE IS A RULE OF THUMB FOR AIR MOVEMENT: If you do not know what the CFM rating is, (you need this for the correct air exchange) use this method: Multiply your room length by the width. Then, multiply by the height of the room. This is your air volume. Divide the air volume of the room by the CFM rating on your fan. You want a complete air exchange atleast every 5 minutes.