This depends on the type of screenplay your wanting to write. A treatment screenplay doesn't have to be very long at all it just needs to give the reader/screenplay buyer enough information to be able to produce the movie. A full script is a little different.
I think he's even thought most script agents expect your screenplay to be a certain length. I can recall telling one agent how long my screenplay was and then afterwards he said normally screenplays are longer than that. That screenplay was 68 pages. Although it wasn't many pages I added a scene description after each scene which described the view and other elements which would even out the script. So I think you should at least shoot for one hundred if you can. Writing a screenplay is not like writing a novel or short story description is everything.
