Everybody has their theory but this is the way I go about it.
I shoot CO2 in the veg room 3 times during the day cycle none at night where it will be just a waste of money. When no CO2 is being run I like my veg room to be about 70F and 65-70% humidity. With CO2 I will take things up to 80F-80%. I believe that the CO2 is not effective unless you up the other conditions as well. It's kind of like if you up the water and not the light, nutes, and CO2 (fresh air normally) your wasting water. A schedule will run something like this:
00:00 Run CO2
00:05 Stop CO2
00:50 Run vent system intake and exhaust for complete air exchange
01:00 Stop vent system and start fans
03:00 Run CO2
03:05 Stop CO2
03:50 Run vent system intake and exhaust for complete air exchange
04:00 Stop vent system and start fans
You get my drift. I like to let the CO2 sit with no fans for awhile to let it soak in. While running this schedule I up water and nutes. I also make sure where I would normally be running a 600 for lighting to run a 1000 instead. In this way I have upped everything needed by the plant in proportion with each other. Some of you may agree and some may not but I see about 25% more growth by following this plan than without it.
Good luck with your grow.