There’s a rule you’ll see over and over again in script writing that states that one page of script is about 1 minute of stage or screen time. You’re told that a 30 minute play should be 30 pages or a 90 minute movie would be a 90 page script. But while it’s better than nothing at giving you an idea of how long your script will be performed, it’s not the ironclad rule you’ll see it presented as.
For example: I have never seen a production of The Love of Three Oranges that ran under an hour and a half (heck, in some productions that’s the length of Act One alone) and the Playscripts version of that script is 47 pages total (that’s just the actual script part minus all the front and back matter). When you move that same content into 8.5 by 11 standard letter size (which is how most of us write), it’s only 30 pages long. That means each page translates to an average 4 minutes of stage time because of all the improvisation, fight scenes, comedy bits, etc.
The lesson here is that the final run time is going to really depend on your script. A 30 page script could be a quick one act or, like Oranges, it could be a full length play depending on how much of the script is dialog and how much is action without anyone speaking. Saying “they fight” is only a single line in a script but it could be several minutes of screen or stage time. Leaving room for improvisation in your script is also going to lengthen the time it takes to perform.
Well then how do you know how long your script really is? Get some actors together and do a reading of it. Or, better yet, stage an actual production of it. Taking it off the page and onto the stage or screen is really the best way to know how each individual show translates into pages per minute.