Personally, what I find works best is 3 or 4 sessions. This should give you ample time to go over character choices and blocking. Use the first day as a read-through with as much of the speaking cast as you can get...read through it once without stopping (have someone dedicated to the non-dialog text--preferably a good reader), then read through a second time stopping people with ideas and specific choices. The rest of the days should be broken down for dialog-heavy scenes. Work key scenes you think will need it.
For most of the blocking, and smaller scenes, you can do all that on set the day of shooting. If you're hiring quality actors, they will come prepared, off book, with character choices and ideas, as well as be malleable enough to fashion on the fly.
For stunts or larger action scenes, you should hold separate rehearsals.
My example is for a feature...for a short all of this can be lessened.