How Much Time It Takes?

I am one of your scholars who wants to write a screenplay.
I heard you talk about allocating specific time to complete a task and I agree with you that it makes sense to do that when you know exactly what you are going to get done.

But if I set out to write a 100-pages screenplay and I do not even know yet what the story will be about, how can I implement your idea?
With writing fiction, we have no clue as to how long completing a story will take.
If it were so, writers would complete books and screenwriters would finish screenplays over a single weekend, but that is not happening…

Would you mind explaining how to incorporate that to writers of fiction?