I use Co2 only in my flowering room 3x's on 15 min. intervals ONLY when lights are on you can give your gurls Co2 poisoning if on during the dark phase because they need the light to make use of it,how much Co2 depends on the size here I found this
Co2 Calculations:
1. Measure and multiply: length x width x height of your grow room to calculate the volume of cubic feet
2. Generally, the c02 level in a grow room is 300 ppm. To bring your room to an optimum level of 1 500 ppm, you will need to raise the c02 by 1 200 ppm.
3. Multiply your room volume by 0.0012 (1 200 ppm = 0.0012) to determine how much c02 to add to your grow room.
EXAMPLE: a 10' x 8' x 10' room:
1. Volume= L x W x H
volume= 10 x 8 x 10 = 800 cubic feet
2. 800 cubic feet x 0.0012 =.96 cubic feet-- you can round this to 1 cubic foot. You will therefore need to add 1 cubic foot of c02 to a 10 x 8 x 10 grow room to bring the c02 level to 1 500 ppm.