Screenwriter David Skaufjord wrote a long, unedited but quite practical post about how he wrote a script in 24 hours.
After exploring his unorthodox method, you can read The Karate Kid-themed script he produced during the writing marathon. The post includes some invaluable advice about screenwriting structure:
One of the questions that get tossed around the most when it comes to screenwriting, is how long it takes to write a screenplay. Speaking from my own experience, I´ll say that it takes somewhere between 24 hours and 3.5 years. Now, how long it takes to make a good one is a whole other scenario. But most “writers” you´ll run into never actually finish one. Here´s how I wrote a screenplay in 24 hours. I´m gonna make a habit of forwarding this to every person I run into who claim to carry around a great story, yet don´t have time to write it.