How long is a piece of string?
It varies.
How much leaf surface area in the room.
Leaks in the room.
etc.
But I'll tell ya they use it fairly quickly. I would go from 1500ppm down to 800 is about 20 minutes. (did have some leaks an such) that was six plants of about 2.5 foot tall.
Without a meter, its all guess work.
with a meter its educated guess work.
With a closed loop controler, its spot on.
CO2 is the last thing to add to a grow op. Have good lights, and temp and stable consistent grows before using CO2. And for what its worth, the indoor CO2 here is twice that of the outside. (give or take 50ppm)