It stops being angry only when nobody else is around. To view this moment is very difficult.
The Pokémon is full of vitality, and that prevents it from falling asleep.
The Pokémon is angered when it takes a critical hit, and that maxes its Attack stat.
Hidden Ability
If the Pokémon has any stat lowered by an opposing Pokémon, its Attack stat will be boosted sharply.
