i All commercial cabins pressurise to about 7800 to 8900 feet. So if the aircraft is above this altitude, this is what the passengers get. This effective altitude can be reduced, and we do so in air ambulances, but it is not offered nor practiced on commercial aircraft.
Most passengers will desaturate to the low 80s on a pulse oximeter. This has no adverse effects. The monitor is not linear and has multiple variables so unless you are a doctor I would throw it away.
If you have significant emphysema or get breathless in flight, you need to take medical advice. Otherwise forget it.
Loss of consciousness is very interesting - with massive decompression you lose consciousness in about 10 seconds - the CAA say 2-3 seconds but this is not borne out from practice. The reason is not only the loss of oxygen but multifactorial
In gerenal low oxygen levels produce symptoms similar to drunkeness. The level required for LOC will depend on the individual and rate of decline. 50% oxygen is 2.5 times normal, but 50% of normal is not enough