There are plenty of books and other material on screenplay formatting and specifically on how long a screenplay should be according to genre.
But WHY is this important?
Well there are several answers to that.
Over the last hundred or so years of filmmaking the business model has been built around how often a film can be shown in a theater per day. Looking at this and nothing else, it stands to reason that shorter films can be shown more often and therefore generate more revenue.
But too short and moviegoers will somehow feel cheated. By trial and error most films have been edited to 90-120 minutes in length. Genre does have something to do with that but also the film's story and pacing have more to do with film length.
For example, looking at the James Bond franchise of films, we all know what genre it is, and what the basic story line will be. Yet, these films can have quite different lengths.
The 2012 release of the last 007 film starring Daniel Craig, Skyfall clocks in at 143 minutes, that's over two hours of action fun. Yet, the 2008 Quantum of Solace was merely 106 minutes in length. The 2006 re-make of Casino Royale was 144 minutes while the original 1967 version starring Peter Sellers was 133 minutes.
WAIT! Did you say Peter Sellers? The same dude from The Pink Panther? Yes, it seems even Mr. Sellers donned the shoes of 007.
So what does that have to do with writing?
You need to be aware of what an audience is going to accept and what they will be able to handle. Horror films seldom go beyond 90 minutes because we can only take so much blood and guts. Thrillers and Suspense is also a genre that usually fit well between the 90-100 minute range.
You as the screenwriter should be aware about how long similar films are in the genre you are writing. This somewhat helps you as the writer organize and plan the plot turns and sequences which should give you a stronger pacing.
Last, be aware that films can and will get edited. So giving a few extra pages (minutes) to your story also helps. Instead of forcing your horror script to end right at 90 pages, consider giving it a few extra pages... say 94-98 so that there is some room to edit.