The gas should be good for at least three weeks, or three days past forever. Nobody really knows, but I routinely put aside fills for planned dives that I use a year or two later. If they analysis matches what they were put away with, I don't worry. Never have I had one *not* match, but there's always a first time, I suppose.
From a safety standpoint, aluminum cylinders should be stored either full or with a few hundred PSI in them but no more. Otherwise, in a fire the walls can lose strength before the burst disc blows and the tank can go "boom." Or something (I have never heard one go in a fire, but I'd imagine that one with air would just make a big "whoooosh!" noise.) Lest you think this is mere theory, I saw this tank in a Florida dive shop a few years ago...you will note that if the burst disc had blown, the tank would be intact (though still destroyed):
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Steel tanks should be find either way. It's hard to imagine the tank getting hot enough to blow before the disc goes from heat or pressure unless the heating is very localized (think "Oxyacetylene torch").