fresh air needs to be replaced mulitiple times a day. i would exhaust the room probably every 15 mins. or so. i keep my fan running constantly. read up on ventilation here, you'll find a lot of useful info if you just look around. are you using any additional c02? if not then you can keep your fan on continuously. In an unsealed room, the source of
C02 comes in everytime you exchange air in the room for outside air.