Hydrostatic testing should be done on scuba cylinders every 5 years unless something has happened to the tank in the meantime that would indicate a need to check the tank sooner. Since the hydro test stretches the metal of the tank beyond what a normal fill would do, it is not recommended that the test be done more often. In any case, most scuba tanks are guaranteed for 100,000 fills if they have been visually inspected once per year and hydrostatically tested once every five years.