If the plane is falling out of the sky at a 45 degree angle the air is being sucked out of the plane and your little oxygen mask falls down will you be able to reach it if you've got your seat belt on?
Furthermore, If the airplane you're on is going to crash why do they try to stop you from passing out by giving you oxygen?
Maybe they don't supply oxygen at all, but are only there to muffle the screams.......
I wouldn't have thought it would take 10 minutes to fall to the ground? If it does, then it's going to be far enough down for us to breath properly (between screams)?
10 minutes of oxygen is enough for the plane to get from its cruise level to a level where you can breath without the aid of oxygen, FL100 or 10,000ft is the nominal level for such things.
It's not meant for you to use for a long period of time as it's impractical and uneconomical for planes to carry large amounts of oxygen due to space and weight restrictions.
Any airliner flying today will be able to get from its cruise level to 10,000ft before your oxygen runs out so don't worry.