In my book I say a movie is a short story, not a novel. I also say it's a simple story. Walter Lee of Sydney, Australia asks: how simple?
I was going to ask "how long is a piece of string?" but I realized that I actually can
answer this question, a couple of ways:
Beats: In my experience, a movie is between forty five and sixty steps or "beats." A beat is a unit of storytelling in which something happens. It could be a fight, a revelation, a chase, an argument, etc. In my experience a beat usually becomes a two page scene; longer scenes usually have a dramatic turn of some kind within them where something new happens and you're into a second beat without ending the scene. These days I find I'm writing more compressed scenes, so practically, sixty beats winds up giving me a script of about a hundred pages. Your studio-oriented scripts probably tend towards 120+ pages with two-minute scenes, and once the director gets on board they swell to 130+ pages, but I like to shoot for 100 pages and I rarely consider a script polished that is over 110.
My last script had 58 steps. The rough draft came in at 127 pages (ack!), the first draft came in at 115, and the second draft is 97 pages. Which is the perfect length for a comedy, according to Woody Allen.
Memory: A more organic way to tell if your story is the right length is to tell it out loud -- which you should do anyway. If you can tell your entire story, beat by beat, in between five and ten minutes, it's the right length. Shorter than that and it's either too short, or you're describing
the story rather than telling it. Longer than that and you're either fleshing out the scenes too much in the telling, or your story is too involved. Also, if you've told it a bunch of times and you can't remember every beat, it's likely too complicated.
That said, if you know what you're doing, you can break these rules, but that's true about any screenwriting "rules".