The writer Syd Field has identified what he calls the ideal Paradigm three act structure.
In this structure the film must be set-up within the first 20-30 minutes before the main character experiences a 'plot point' that gives them a goal to achieve.
Approximately half the movies running time must be taken up with the characters struggle to achieve their goal ; this is the 'Confrontation period'.
Field also refers, sometimes to the midpoint, a more subtle turning point that happens in Act 2 - the confrontation which often has an apparently devastating reversal of the main characters fortune.
The final quarter of the film(third act) depicts a climatic struggle by the protagonist to finally achieve (or not) their goal and the aftermath of this struggle.
