A rule of thumb has been in existence for a rather long time which states that “one page of script equals one minute of screen time.” Whilst this method isn’t overly scientific it has a strange characteristic of being true most of the time.
Obviously whether the one page per minute rule works for you is of course dependent on the content your script. For example, a page of dialogue will most like represent a completely different amount of time on screen than a page of scenic description. And on top of that, the decisions made by the director in how they visualise the script will also have a major effect on the final running time. A good example of this involves the “Staircase of Baradur” sequence from the first film in the Lord of the Rings trilogy. In the original script, this sequence is described in a single line (paraphrased): “The fellowship runs down the stairs.” In the final film, director Peter Jackson decided to turn this sequence into a major set-piece and the final screen time ended up being several minutes with the companions struggling to get across as the staircase collapses.
So whilst it’s useful to use the one page per minute rule of thumb as a rough guide, it’s better to either have your script timed by a professional, or at least for you to time yourself reading through it (aloud) at a realistic pace.