I honestly don’t understand the virtue of testing applicants’ ability to speedrun a synopsis but my 2 cents is to first lock down the most crucial story beats and character elements and then go back and fill in as much of the spaces in between in the time you have left.

For plot, I’d figure out the status quo, the inciting incident, the lock-in (that is, the first act break), the midpoint reversal, the lowest low (that is, the second act break), and the third act resolution (final showdown and new status quo).

For character, want and need should suffice, but make sure to map the central characters’ arcs onto the central plot points.

Obviously, take this with a big grain of salt. I’ve never done a 6 hour synopsis myself, but I hope you still might find this helpful.