It is relative to the local laws for the country the cylinder is used
The U.S. Department of Transportation rules require an annual visual inspection of all scuba tanks. UK Visual Inspection (every 2.5 years)
Essentially this test is a detailed visual inspection.
i would ask? what if the tank was used at a dive centre?
How many times would it have been filled in six months? surely it should be inspected more frequently? now thats the MILLION DOLLAR QUESTION!
MY ADVICE: at a minimum a visual inspection annually should be conducted on tank.