@bobmorane 's method is certainly valid.
In addition, cylinders almost always use regulators which almost always have pressure gauges. You know how much gas the tank originally contained by it's specifications, including volume and its original pressure, which are both almost always industry standards. And you know how much gas you have left by measuring the current pressure with the pressure gauge.
Remember, according to the recommendations of the PPH, you need a minimum of 15 minutes at 15 liters per minute of flow. Or, you need an absolute minimum of 225 liters of uncompressed argon remaining.