Honestly, in my experience at least, the book is way off when it comes to its estimations of how long sessions take in-game, how long a scenario lasts, etc. The book seems to take it as a given that any one session is going to encompass several days of activity, while most of the games I've ran, a period of two or three days might take a dozen or more sessions, with each session picking up immediately where the last left off and the occasional timeskip of a handful of hours. According to the book, that would mean a character recovering from a bad concussion over the course of a couple hours instead of weeks, or Harry being able to use his force rings much more frequently than the canon suggests.
So while it may be a useful guideline, take it with a grain of salt.
My general guideline has been to have Significant milestones after mid-boss fights or some other equivalent accomplishment, Major milestones at the end of a Scenario.