Typical high pressure steel cylinders for nitrogen, argon, etc every 10 yrs.But in response to capz I never heard of a hydro on a tank longer then 5 years.. I mean air is not corrosive and that needs to be done every 5 years..
§ 180.209 Requirements for requalification of specification cylinders.
(a) Periodic qualification of cylinders. (1) Each specification cylinder that becomes due for periodic requalification, as specified in the following table, must be requalified and marked in conformance with the requirements of this subpart. Requalification records must be maintained in accordance with §180.215. Table 1 follows:
(b) DOT 3A or 3AA cylinders. (1) A cylinder conforming to specification DOT 3A or 3AA with a water capacity of 56.7 kg (125 lb) or less that is removed from any cluster, bank, group, rack, or vehicle each time it is filled, may be requalified every ten years instead of every five years, provided the cylinder conforms to all of the following conditions:
(i) The cylinder was manufactured after December 31, 1945.