When I am writing I don't set a certain number of pages. I do know that the further into a script I get the faster it goes. As soon as you start making decisions you start cutting off all of the other possibilities of things that could happen. So with every decision that you make you are removing a whole bunch of other possibilities of where that story can go or what that character can do. So when I get maybe 2/3's of the way through I can see very clearly where it is going to go.