How much CO2 is needed?

1st of all CO2 needs recovery time to boil and build pressure back up. So go big on the plenum. I think it would be about 



maybe....if... 20 decent fast fired bb shots from 1 cylinder, before resting

200 shots without stopping could be 10 cylinders

I wasn't thinking a continuous stream. More like bursts between 10 and 20 with no reload.