This topic came up on PPRuNe some years ago, not Airbus specific, but covering all types. The general consensus amongst Prooners after quite sensible discussion went something like this -
Descend to 25,000 feet. At 25,000 feet Time of Useful Consciousness is 4 minutes, more than enough to descend rapidly to 10,000 feet following depressurisation. (It is also why aircraft not operating above 25,000 feet do not require a passenger Oxygen system).
At 25,000 feet, evaluate your range capability, and continue to destination or divert to Alternate as necessary. Don't forget that all flights must carry sufficient fuel for diversion to Alternate at 10,000 feet if depressurised, so flight at 25,000 feet will be well possible.
Where you're in trouble is crossing extended areas of high terrain with high MEAs, Afghanistan, Iran etc., where you might have to spend some time at around 18-19,000 feet, in which case carrying 2 of the portable Oxygen bottles in the cockpit is advised.
I've actually had a total loss of cockpit oxygen, and I chose to descend to F/L 250, if memory serves me correctly,
411A reported a similar experience.
Oh
Hetfield, why did you have to bring up smoke? Screws up all the theories
Regards,
Old Smokey