When can you take the mask off?

[Yukari Yakumo]

Ok so big purple is in your room looking to itemize your deductions so you slap on the fazbear mask.

When is it safe to remove the mask?

[ChaosConvexum]

I'm assuming I'm big purple. Anyway, it's safe to remove your mask as soon as "Big Purple Smexy Butt" disappears from your vision... assuming you slapped your mask on in time.

[Pepman]

What Miss Bonnie up here said. The lights will flicker for a few seconds, and if you didn't screw something up, you should be able to take the mask off without getting a free scream in the face. However, if you have been pretty lucky with animatronics not coming in, you'll probably not have quick enough reflexes your first few times, as you literally have to put the mask on half a second after one comes in.

[Teal Turken]

I found that you can remove the mask while the light is returning after the attacking mascot leaves.
I recommend using your first few seconds of freedom outside the mask to wind the music box.

[Silvy]

The Game doesnt let you take off the mask untill its safe (really nice feature, since it saves time and makes it less annoying) so just keep trying, you can take it off quite soon as yoy can.