Mickjoebill
Crew O2 has 2 setting continuous flow or on demand. How long it last is relative to how hard you breath and the setting selected. And each individual is different. If there is a decompression in in the cabin. One of the priority of the flight crew besides putting on their O2 mass is to reduce their altitude as fast as they can to get to an altitude where they do not need to depend on having oxygen. Believe me the 1800psi do not last as long as you think if you are breathing fast and hard. (how fast does your 32psi tyre deflate?)
The pax cabin has 12 to 16, (depending on configuration) 1800 psi bottle all connected together to feed the whole pax cabin. It only supply O2 when the mask is pulled towards you and not when it is just hanging down. How long is last is relative too. A fully loaded aircraft will use the O2 faster. So they are relying on the flight crew to get it down to a breathable altitude asap.