I think it would be helpful if you asked how many play sessions it took groups to complete each section.
It took my group three play sessions to finish Part 1... and we skipped the boss battle (the GM just narrated it rather than playing it out.)
We took four sessions to finish Part 2, and the GM decided to skip the fight with the Night Heralds.
We have not started Part 3 yet.
Honestly... we are not having all that much fun with the playtest.