Thabo,
You're talking about 3 minutes to get down to an altitude where people could breathe the outside air. People can go from anywhere between 3 - 10 minutes without oxygen without suffering brain damage. Given that as the plane descends there will be _some_ (ok, not much) oxygen around, people would probably be ok. I assume you're talking about a worst-case scenario where oxygen masks have failed to deploy for some reason.