As a general rule of thumb, it takes about a ½ Lb of CO2 to dispense a ¼ barrel of draft beer and 1 Lb of CO2to dispense a ½ barrel of draft beer. This table lists the approximate number of kegs that can be dispensed from each size of CO2 cylinder.

	2-1/2 Pound Cylinder	5 Pound Cylinder	10 Pound Cylinder	15 Pound Cylinder	20 Pound Cylinder
Home Brew (5.00 Gallon)	7-11	15-22	31-44	46-66	62-87
Sixth Barrel (5.23 Gallon)	7-11	14-21	29-42	44-63	59-83
Quarter Barrel (7.75 Gallon)	5-7	10-14	20-28	30-42	40-56
Half Barrel (15.50 Gallon)	2-4	5-7	10-14	15-21	20-28

 

A gas tank that is properly filled should read about 750 PSIG when the cylinder is at a 72°F temperature. CO2 gas pressure increases when it is exposed to heat, so if the gas tank temperature is above 72°F, the pressure will read higher than 750 PSIG. If the gas tank temperature is colder then 72°F, the pressure will read lower then 750 PSIG.

The tank pressure should read 750 PSIG until the liquid CO2 in the cylinder is gone, and the tank is almost empty. A decreasing pressure indicates the cylinder is nearly empty.

 

Ref: http://www.micromatic.com/beer-questions/how-many-beer-kegs-dispensed-out-co2-tank-aid-89.html