The plants use Co2 to help during the photosyntheses process. So if the lights are off, you won't need it. I'm not really sure about the whole time thing. I just read somewhere that with a small grow like mine and such a small space, you don't need to let it run too long. So maybe like 10 mins every four hours?
Here, read through this:
Growing cannabis ( marijuana ) with C02
Don't get caught up on all this though....it's just another added feature..