Most CO2 cylinders have regulators with dual gauges that allow users to check the pressure of the CO2 gas in the cylinder. Regulators that do not have dual gauges have a sticker-type device that allows the user to check the pressure in the tank.
How quickly the CO2 cylinder empties depends on the size of the cylinder and the rate at which the gas is used. Many users feel that when the pressure gauge reads about 600 pounds per square inch, it is time to get a refill. Users with a spare cylinder can wait for the pressure to go well below this reading.