I used to get at least 4 weeks out of this setup on a 46 gallon aquarium. My bubble count was high enough to make it difficult to count. I had 2 20 OZ and 1 24 OZ paintball cylinder. When I got to the third one, I'd get the other two filled. I only did this because of space constraints. Since I switched to a larger tank and stand, I also switched to a 5 lb cylinder.
If you do use an inline bubble counter, I advice using two check valves, one on either side of the bubble counter. Sometimes when the solenoid would shut off, water from the bubble counter would get in the tube leading to the solenoid.