Mei is a character who freezes things. It's what she does. Freezing enemy players is an effective way to immobilize them and land a perfect headshot (killing many of the lower health characters). Typically, the way an encounter works is that you'll spray them with your primary fire, and then headshot them with your alternate fire once they're frozen.
This doesn't leave you with a lot of time, though. Once a character his the freezing point, there's an animation of their body being covered in ice. When exactly are they considered frozen (i.e. I don't need to keep spraying them), so I know to begin headshot protocol?