Everyone is different in their writing process. Some of us write for hours and "crank it out." Others write in chunks. And others have a hard time even starting page one. This tip supports the checkpoint (chunk) writers.
Chunk writing is a great place to start as a screenwriter. A screenplay is first a blueprint. When a it is edited is when it turns into a story. First drafts are to get the A to Z out of our system. But, this is hard. Designating specific stopping points will help get initial pacing out of the way. Don't put all faith in this first step, though, as this pacing will change. That's a promise.
Remember, structure is important. Yes, certain actions and transformations should be happening at certain spots in your script. But feel free to bend that traditional model in ways that are coherent. Coherency, over anything, is most important. One of the best ways to obtain this coherency? Write. In. Chunks.
The next time you sit down to outline, draft your first version or edit your final draft, make sure to stop after a certain amount of minutes (twenty to thirty minutes tends to be a good range). Don't apply this tactic to page numbers because that can potentially take much longer than a "chunk of time."