I'm not sure what the tank should be set for for cherry shrimp..maybe one bubble every two seconds? If that is even the correct way to measure...Or do you just mess with it until the drop checker turns green? But how long approximately would a tank last when set correctly?
Bubble rate is not the best way to measure CO2, since bubble size can vary.
Using a drop checker with a 4 dkH reference solution will allow you to estimate approximately 30 ppm of CO2. From there, you need to watch your livestock to ensure that your levels of CO2 are good (sometimes you will find that you can increase the CO2 more, other times, you will need to decrease your CO2).
I would say a paintball cylinder should last at least a few months on a 20 litre aquarium. If you meant a 20 gallon (long), then maybe a month or two.
I have two 20 oz PB one on a ten gallon and one on a 20 gallon and I run both of them 24 X 7 and keep the drop checker in the green. The one on the 10 gallon will last aproxmatly 2 months and the one on the 20 around a month. I'm using PB because of lack of space and the tanks are not close enough to share a large one. The PB only cost me $5.00 to fill.
I also have a 45 that I run a 24 oz PB on and again becasue I dont have space where it sets for a big CO2 tank it sits behind it and it will last me 1.5 to 2 months.