When I first started out writing screenplays, I learned that you need to hook the reader within the first ten pages.
You need to at least introduce the main character, and tell the reader what the story is about and what to expect. Your target audience is that reader who gets paid to read twenty scripts a weekend. If you want him or her to pick out your script, make sure your first ten pages keeps the reader wanting more.
Sometimes it may take twelve pages, but try for ten.