It depends really, short if your doing like a short story type of thing, but if you want to get people to really relate to and develop the characters then make it fairly long I'd say :)
The average would be no less than 105 pages and not usually more than 120 page, but you should also break it down into acts. Consider about 30 pages per act.
Short enough to be interesting.....long enough to encompass the storyline....
That depends on what your script is for.