Alright sir, if you don't get on oxygen you're going to die. [YT] Destin Sandlin of "Smarter Every Day" goes into a high altitude simulation chamber where he demonstrates why you should definitely, always, put on your own oxygen mask first. The chamber is set to the equivalent of 25,000 feet (7,620 meters) after a slow decompression, which allows 3 to 5 minutes of consciousness. The video points out that "most airliners travel at 35,000 feet" (10,670 meters), and under a real scenario you may get only 15-30 seconds of useful consciousness.
Remember the woman arrested for allowing her kids to be sunburned at a county fair? What's she doing now? Sadly one of her children has died and it has been ruled a homicide.