Several weeks - even up to months. I usually have the idea and then I let it simmer as I work on other projects. Getting closer, I outline those 9 beats, figure out the character names, and let it simmer again. From there, I fade in. As noted, I work best having a vomit first draft down and then spending the majority of my time revising and rewriting.

I used to take barely a few weeks to prep, leading to a load of problems with my scripts. Now it's boiled down to about six to seven weeks. Once I have the idea, follow a lot of what Scott suggests - I write character bios, also like to research, write a treatment of the story to get my creative spirit boiling. Then I head into a very very long outline/beatsheet and write everything, even if it seems meaningless. Then I head into first draft and basically, just like Lisa - vomit it without looking back - so prep itself systematically now takes me about 6- 7 weeks and writing the first draft about four. Rewriting takes much much longer...