Concorde climbs to a normal initial altitude that most heavies operate at and immediately begins a slow rise (aided by fuel and therefore weight loss) to the peak of it's Mach II cruise, generally about 60,000 feet. Upon reaching peak altitude, the aircraft slowly begins descending back down to it's original intial altitude where the pilots will either wait to descend for landing or begin the descent immediately. So even if there was depressurization in flight, chances are very slim that it would happen at that particular point in time. If it did, you're right -- a Mach II descent wouldn't take long at all. I shouldn't think the size of the windows would be a factor really -- the windows are still big enough to let a depressuriztion happen in less than 10 seconds, in order to be a factor it would have to be puncture-hole size.
So the answer to the boiling question is: "virtually impossible". Also, I don't belive the captains are required to wear oxygen for any length of time during flight bar any kind of emergency.
Hope this helps.
SFly
