The rule of thumb is that you decide whether or not you like a film within the first ten minutes of the running time. This must be the reason for the screenplay "hook" which is supposed to grab the viewer and keep them watching. A classic example of this is the opening sequence of most Bond films. It's always an elaborately staged action sequence that typically has nothing to do with the rest of the movie. It's an attention-getter that says "what comes next is just as good!".
The sad truth is that many movies have great hooks, but few deliver what they promise. I can't count the number of movies that hook you then follow with a good setup only to sag in the middle, and fall apart in the end. Good ideas are aplenty, but fleshing them out to a feature length is tough, especially if your are writing your first full-length script (like me).
My favorite movie hook is probably the opening sequence from The Matrix (1999). While it is a wonderful and thrilling action setpiece, it is more than just screen viscera. It creates all kinds of questions in the watcher (How did that girl run up the wall? And disappear into the phone? And who are those "men in black" types?) then spends the rest of the film answering them. This is what we all want: to create a void in the audience that will keep them interested in how everything pans out.
I hear action producer Joel Silver demands an action sequence (or hook) every eleven minutes. This is not a bad idea in any movie. It doesn't have to be action, but the more hooks you can put out, the more interest will remain. A police taser is a submission weapon that shoots two wired barbs into a human target followed by an electric charge. As long as those barbs remain attached, the officer can continue to deliver the shock. So it should be with our script. Keep pressing that button and you will keep eyeballs on the screen.
What these hooks are, of course, will depend on your story. Comedies will have much different hooks than a horror film or western. It's up to you to know your characters and plot to implement bits that intrigue and perplex. Just don't back off when the barbs go in. Keep pressing the button all the way until the final credits roll.