It can happen dead smack in the middle of Act Two.
It can occur at the end of Act Two. Or leading up to the climax in Act Three. Maybe somewhere else entirely. And often more than once.
The hero has to face their own personal hell, their own worst fears writ large. It doesn’t mean flaming devils and pitchforks (unless they’re, say, Mike Pence). It means the place they least want to be. With the people they least want to be with.
It could be sitting tightly between two sweaty fat people on a plane that never, ever lands. It could be stepping down into the dark cellar where the psychopath is filing hos teeth into fangs. It could be waking up in a coffin freshly buried in the damp autumn soil.
Space and time become constricted. A narrow gate, a gauntlet, a crucible.