How short can my screenplay be?

Generally speaking, as a novice your spec scripts should be between 110 and 120 pages.

The rule of thumb with screenplays is that one page equals one minute of screen time. Of course, this is not an exact science, nor do all movies run between 110 and 120 minutes, but keeping your screenplays in this range to start will help give them a more professional feel.

As you become more established, you will have more freedom to bend the rules.