Rip the mask? Do you mean a SCBA mask? That's impossible, it has 5 extremely strong attachment points, you can't remove that while being unconscious. Even when conscious it takes a good amount of finesse and precision to remove it.
On top of that you have a second layer of protection: the positive pressure of the demand valve, which is capable of covering any leaks by increasing the pressure inside the mask. Even if you put a finger inside the mask the demand valve would cover the leak and won't allow any external air to enter (it would use more gas tho).
If you change you mind you have to unplug the demand valve from the mask (which also requires precision with both hands at the same time), not trying to remove the mask (you would blackout before being able to even loosen a couple of attachment points).
Loss of consciousness in a 0% oxygen environment happens in about 3 breaths (assuming you have pre-filled the mask and exhaled all air from the lungs before taking the big deep breath of N₂). That's what happened
to @Diver_K_A and the subjects in the
Russel Ogden's study.
@Wunderkind also blacked out during his tests without even noticing.