rather WHEN you exchange your air, the exhaust fan should be able to cycle it in 3-5 minutes
take the volume of room and divide this by 3-5
this should be the minimum rating of exhaust fan you should have.
You want this to happen when room temps get to hot.
However, many people will run their fans on low constantly also to help manage scents (assuming they are using a carbon filter etc..)
Another common practice is to cycle the room once an hour.
For my space I'm setting it up to always run on low. And then cycle it once per hour, and also to kick in when temps get above 83