If nos bottles are like the ones I use for work, there should be a date stamped on the bottle letting you know when it will need a hydrostatic test. As Steve said should be 3-5 years depending on the bottle type
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Should be the same! I know it's also the same for scuba tanks
But, Lee, you fill your own bottles. So it shouldn't be a big issue for you.
It's only places that fill that check