how much co2 can a plant absorb in a hr/day

I can't really answer your question, in those terms, however:

1. The optimum CO2 mixture is about 1,500 ppm (Parts per million), during "Lights-on" only.
2. The higher you go, above 1,500 ppm, you will start to experience "Diminishing Returns" (less "Bang for the Buck").
3. At some point the CO2 level can get so high, as to become toxic to the plant (I'm not really sure if high CO2 levels are actually poisonous, or if they just prevent simple, elemental, Oxygen from reaching the roots by exclusion).

Carbon is actually one of the required nutrients - but it is absorbed through the leaves (out of the "air") instead of the root system.

So the answer to your question depends on many things, mostly how big the plant is, just try to keep it at 1,500 ppm for optimum results.

Good link for you to answer anything you need to know.

http://www.weedfarmer.com/marijuana/co2.html

about a dozen plants about 2 ft tall and growin!!in a 10x3x8 closet.. i keep hearing during the dark period is the best to inject the co2... why is that