If you are growing in a tent/closet or enclosed space you want to exchange 8 times the volume per hour used to be the rule of thumb. If you are in a big indoor space I wouldn't worry about that. If you are using odor control with your exhaust you should run it 24/7. Other people choose to run fully sealed grow setups and use co2. There really are many ways to do this, you just need to decide what your needs are.