There is a saying, and it is mostly true, that you have to write a million words before you're ready to be published.
A million words is between three and four thousand double-spaced pages.
Most of the authors I know sold their fourth or fifth attempt at a novel. Me too. That means we had three or four trunk novels before we sold. Of course, we didn't write them to be trunk novels. We wrote them to be bestsellers. But we were learning. We're still learning, and we still sometimes produce trunk novels.
Recently I ran into a writer who said he knew he had to write a million words before he'd be publishable, and he figured it would take him 18 months at 2000 words a day. This reminds me of my approach to a PhD. See, I was once in this PhD program for some reason. And I kept calculating how quickly I could get out of it. And people who had been through it looked at me in dismay and said, "You're missing the point."
I didn't get the point till I ran into that writer.
The point, in any learning we do, is the process. Not the product.
I think we learn more from revision than we do from the initial writing. If we merely crank out a million words without stopping to look at them, analyze, recognize where we've gone wrong and what we need to do to fix it, we'll end up not much better off than when we started.
The million words are incidental. A means of trying to quantify just how much there is to learn. Unless we're present in the moment, fully focused on the process, on recognizing our errors and learning from them, we're not going to learn at all.
If you're new to writing and are planning to do NaNoWriMo, go for it! You'll be 50,000 words on your way. And once you've spent a year revising and re-rewriting your NaNoWriMo project, you may well be 250,000 words on your way.
(By which I mean not that you should write a 250,000 word novel-- you shouldn't!-- but that the writing done in revision is part of the million words.)