BANG, 19 minutes in the film, a body falls on Max's windshield. Shock! Horror! Nineteen minutes into the picture!!!! Cannot be done. Too soon. Syd Field's Screenplay says the first plot point absolutely cannot come until 25-27 pages into the script, which at a minute per page would be about 26 minutes. The falling body is EARLY. Which may be part of why it is a shock to audiences, even though they may never have heard of Field. As mentioned in the previous chapter, Field's paradigm is so popular in Hollywood, and so religiously followed, that an audience can tell the rhythm of a film if it has been made under Field's spell.
Field's spell? This falls under the section "The Not-Quite So Good" of Stempel's book so I will assume he is not being facetious. This tyranny of the page numbers is something one would expect from screenwriting books written in the 1990s, not from one written in 2008. No one in screenwriting follows page numbers anymore--especially when there is a better, more comprehensive understanding of how and why Acts turn.
There is no set time when a story must start; no expectation as to when a major plot point drives a narrative in a different direction. Acts turn when the present thematics for a particular Throughline have been exhausted. No later, no sooner. Major Act turns occur when all four Throughlines have completed exploring their particular take on an inequity. This could happen 25 minutes into a screenplay, it could happen 4 minutes in; as long as the writer has sampled thematic material from the Overall Story Throughline, the Main Character Throughline, the Influence Character Throughline, and the Relationship Story Throughline, then the story will function properly.
When and where belongs to the artist. What is a matter of structure and Author's intent.