How do you (roughly) calculate the ratio of script length to screen time? Would you use a different calculation for different genres?
The very general rule is that a page in the script should equal a minute of screen time, which is one reason the industry has standardized around 12-point Courier for the font. Since most screenplays are around 120 pages, the movie should work out to be 120 minutes, or two hours, assuming every scene in the script makes it into the movie.
Of course, a page full of action would likely take longer than a minute, just as a page of rapid-fire dialogue would be a lot faster. That’s why before a movie goes into production, the script is often "timed" to estimate how long the movie will be, so the director and producers can plan accordingly.
A "script timer" is a professional reader who estimates how long each scene will play, and thus, the length of the overall movie. Generally, the script timer will take into account the director’s vision and style when timing the scenes; the David Lynch version of a scene would tend to run longer than the Michael Bay version. Many script timers are in fact the script supervisors, who will be set during the entire production helping the director, actors and editors maintain continuity and catch mistakes. From the screenwriter’s perspective, this is one of the most important people on the set, since he or she always has the director’s ear, and will be the person correcting actors who mangle their lines.