it all depends on room size....most try the whole on for 15 mins at beginning of every hour....then set you fan on another timer to kick off for 30 minutes.
so say
at 1pm your fan kicks off for 30 minutes and your co2 kicks on for 15. You'll need a calculator to figure how much you need to jam in the room.
I jsut took the dive into co2...but I'm not fucking with the timer shit, I ordered the Sentinel Chhc4 controller...takes out all the guess work.
and yes....co2 only during day cycle. No need for it at night.