To answer the first question, one has to look within. Writing takes self discipline, and what motivates us is as individual as we are. There is no right answer to this, so you have to find what works for you. If find solid deadlines work best for me, but for you, a hard deadline my have no meaning. You have to discover your own motivation, create your own routine, and stick with it. Some say it takes 21 days to make a knew habit or break an old one. That's three weeks if that sounds better for you. Certainly not a long time to get into a writing habit.
But what do you do when you get out of the habit? After all, we are only human. The demented elves suggest you go back to step one and look inward, find what motivates you and work on that writing habit again. If you did it once, you can do it again. And no matter what remember your goals.
What difference does it make? The difference is finishing your work or not. If you do not stay on task, you may never finish, or it may take a long time to do so. It also takes you out of your own story and it may take an additional time to remember what it was you were writing in the first place. The demented elves will agree with the experts that say, write everyday. There is no one way to do it, so find the one that works for you and continue to write, write, write!