I'm just getting started with co2, but I'll parrot the wisdom I've heard over the years (some on this very site).

You want to raise the co2 level to some density (generally around 1500-2000ppm, according to what I've been reading) and keep it there for some period of time (some say a couple hours in the early morning, some say periodically throughout the day, and some say whenever the light is on and you can afford the heat of not venting the room). Blasting the plants for a few seconds isn't going to help them, and co2 is slippery shit -- it oozes out of anywhere it can, but always downward. So you need to somehow trickle it in and contain it for a bit. I got a setup off ebay (gas regulator/solenoid and co2 monitor/relay) for about $200. It took me an afternoon to get it all wired up and installed the way I wanted it. I can monitor and adjust it from a computer. It will keep my modestly-air-tight grow space at between 1400 and 1700ppm.

Without gas my grow room is about 350-450ppm in daylight and 550-650ppm at night -- evidence I'd say of the posts here claiming that the plants exhale co2 at night.

Oh -- and for those of you wondering about breathing on your plants -- Exhaling into a paper lunch bag 3 times will raise the co2 level to about 5,000ppm. You can trap 4,000ppm by exhaling into your hands. These co2 monitors are cool!

If you pass out and die in your grow room, you have too much co2. Short of that you don't, and you won't drown the plants.

However, the plants can only use up to about 2000ppm, and as I understand it, they are further limited by the amount of light available at the leaves. I've read 8,000 lumens and 10,000 lumens are about minimal. So a 400w MH blulb is fine, but metered and regulated trickle is still the best way to not waste gas.

I'm just doing my first experiment with co2. After a week of using gas in a 28"x28"x6' area with a 400w MH bulb and five plants, I'm already convinced that $200 investment in the co2 control stuff is going to be well rewarded. The plants show an immediate response to the enrichment, and one clone (the only survivor of a sad story involving my favorite strain) that didn't root so well appears to be rehabilitating itself.