Lester del Rey told me repeatedly that the first and most important part of writing fiction is just to think about the story. Don't write anything down. Don't try to pull anything together right away. Just dream for a while and see what happens. There isn't any timetable involved, no measuring stick for how long it ought to take. For each book, it is different. But that period of thinking, of reflection, is crucial to how successful your story will turn out to be.