Depends on bottle volume and working pressure, consumption rate + reasonable reserve.
Volume stored = bottle internal volume in litres x pressure in bar = total gas stored in litres.
Consumption rate. The old constant flow systems often had only two settings, 2 litres/min and 4 litres/min
The FAA recommendation was 1 litre/min of supplementary oxygen per 10000 feet of altutude so 2 litres/min ought to be ok to 20 000 feet but I'd personally be on 4 litres/min before that!
The new pulsed systems etc are however much more economical on oxgygen than constant flow (4 times or more is claimed). Canulas normally max out at 18000 feet and constant flow is said to max out at 25000.
Depending where you live, your bottle will probably be out of test by now. Diving cylinders in Europe require a visual inspection every 2.5 years and a hydrostatic test every 5 years. I think the same applies to aviation oxygen cylinders but you'd need to check that locally.
All the above is written only as a glider pilot with a lot of diving experience, so check with the professionals. The person selling your demand equipment should know consumption rates and the organisation filling your bottle should be aware of the test regulations applicable.
Last edited by astir 8; 31st Aug 2012 at 03:12.