I've been running my 20# tank for about 6 weeks now. 2hr shots twice a day, running a level between 1400ppm and 1800ppm. Space is 28x28x5'h. I just looked at my high-side regulator gauge and I don't see where it has dropped noticeable from "full".

Your mileage may vary

objects in the rear view mirror are clser than they appear

offer void in the lower 48 and most of the solar system