Depends on what you mean by periods of time but then, you should have
learned that in your scuba level 1 training.

Valid hydrostatic (5 years)
Annual valid visual inspection
air breathed shouldn't taste funny

After this, you must rather insure on the way the compressors where
you fill are well maintained by the station's operator and that he
doesn't fill the bottle with CO, nor oil, nor humidity. Insure the
intake is far from any exhaust source ant that filters (oil and water)
on the compressor are regularly purged and changed. Eg: don't fill 'em
up in cheap holes on the beach. Refill in a responsible and
sanctionned (PADI-NAUI-ACUC-or other diving Intl regulation
organisation) filling station. Your lungs are at stake here. In the
unlikely case of damage from one of a sanctionned outlet, you'll be
covered by their insurance program at least, which might not be the
case for a native running a compressor in hut on a beach in the
Caribeans. Ask for their licence and ask to see the compressor and the
location of the intake.

If at rest for long, I prefer to keep some pressure in the tank to
keep any humidity out of it than having no pressure whatsoever.
Fatigue from the various filling and emptying is what weakens
eventually the tank rather than when sitting unused at whatever
pressure. But you may want to keep them protected from the sun and
temperature variations in the boat though, especially when full. You
will also want them out the way and held tight in place. Tanks and
lead belts swinging in a cockpit can really ruin a gelcoat! That's why
I stopped diving from my own boat!!! ;-)

Jean Dufour
Former PADI Divemaster

padeen wrote:

I'll be cruising with them and they may sit full, then near-empty, for
periods of time. Anyone have suggestions, favorites, warnings, etc.
Thanks,
Brad Snow
s/v Aldonza