I was on BA042 CPT-LHR (G-CIVD) last January when some 2hrs out of CPT the oxygen masks dropped. It was startling!
I was sat in the UD and first thing I noticed was there were four masks/seat pair, so plenty of choice
With the auto cabin announcement blaring, the next thing I noticed was the cabin crew had gone ....back to stations of course....and there was an eerie silence in the cabin (save the auto announcement) and lots of swinging masks
15-30 seconds or so have passed and I have still not donned my mask (young son and missus in row ahead have. Indeed my son put one on his fluffy dog...great picture!) but I was waiting for the nose to shove down....rightly or wrongly, I thought I'd wait
It never happened. After maybe 5 minutes crew emerged and began to engage with the passengers....understandably there were some a little upset
After 15 mins Captain popped up on PA announcing deployment had been a technical fault
I finished my cheese and port and we carried on our merry way to LHR.
On arrival in the morning the lower deck looked like a jungle canopy with all the masks hanging down
I have had altitude chamber training as USAF aircrew. If you wait to see if there is a reason for the mask deployment and the depressurization is real, you will never get your mask on. Time of useful consciousness at airline cruise altitudes is in the 8-10 second range...and you will not have had the advantage of pre-breathing 100% oxygen as we were in the chamber.
Even with a partial depressurization you are seriously impaired in short order. One demonstration in the chamber was at, I think, 25,000 ft. pressure altitude. A student was told to remove his mask, begin a simple task, and not to don his mask again until directed by the instructor. After less than a minute he was told to mask but fixated on the task and ignored the instruction. Not even the instructor screaming MASK at him had any effect - his mask had to be replaced by the instructor.
As a pax, if the mask is presented it's going to be on my face in seconds. I don't care if it looks silly, you get seriously stupid very fast when hypoxic.
This is one reason, I think, why the emergency descent after a depressurization is so rapid. Most untrained people are not going to mask in time to prevent the onset of hypoxia, so the aircraft has to get to a lower altitude very quickly indeed.