A blanket 30% statement made by a flight crewmember is rather unrealistic, unless the skipper or the officer runs through passenger cabin taking a head count of passed out passengers whilst the "Cabin Alt. Warning Horn" and "Emergency Descent" Non normal checklists are being accomplished.

Cabin crew who've experienced a R/D event are better to provide accurate information regarding incapacitated passengers rather than flight crewmembers.

The most common O2 chemical generator systems for many Boeing products provide O2 for about 12. Another system available for Boeing aircraft provide O2 for about 22 minutes.

Airbus may have different O2 generation systems... perhaps Airbus crews can chime in.