Don't assume all types have individual seat row O2 cannisters. Several have plumbed in oxygen systems for pax O2 (all 747s for a start). If indications fell to zero, I would think descent to FL250 would be advisable, but if it was crew O2, why not test it over a long period by one person breathing O2 and seeing how it supplied demand? Periodic test selections supplies prerssure O2? It may be the gauge.