Most CNG cylinders are filled to about 2250 psi and are fitted with regulators that include a pressure gauge. While one could run the gauge to zero, as a practical matter once the pressure falls below 100 PSI one should go for a refill. In many areas it is difficult to find exchanges but sites are available, see SafGas CNG LPG for Boat and Yacht Gas Stoves : Fuel Locations .
A tank about the size of a standard dive tank weighing about 35 lbs when filled will give you about 40 hours of "burner time" on a typical stove-top burner. LPG--Propane--will give you quite a bit more burner time as it has a higer heat value but, of course, is not as safe in use as CNG.