Now that you know it's up to you to decide when you're finished, there's another element to consider: did you write what you were "supposed to" write?
This is an easier question with non-fiction than fiction. For non-fiction, if you're assigned to do a travel piece on the wineries of Italy and you come back with a story about traveling on a fishing trawler in the Barents Sea, you haven't delivered what you promised. In that case your publisher has an easy time saying they won't pay. (Believe it or not, even this isn't always a slam dunk, but that's another topic for another day.)
It gets a bit harder when your writing has some leeway. If you're writing a memoir, profile, or biography, your own impressions and streams of consciousness might be a relevant way to investigate the topic. But the further you get from your designated subject, the easier you're making it for your publisher to decide not to pay you and push you to make your own choice about what happens next.
For fiction it's a much more difficult situation.
If you've promised to deliver a romantic thriller with a female lead set in Renaissance Venice, that gives you a lot of room to maneuver. On one hand, so long as you've ticked all the boxes, you've done it right and you can submit the manuscript. But if you're any kind of creator you've probably been inspired while writing and you've made some creative choices. Did you really stay only in Venice? Or was there some time on the road? How much time does your heroine have to stay in Venice for the story to be set there?
Sure, these are slightly artificial issues, but they mask a real concern: if you're not in pretty tight sync with your editor and your publisher, you risk creating a story that diverges from their expectations. And if you do that you risk not getting paid or ending up in a big fight. As I mentioned yesterday you have the right to decide when you're done, and that means you have the right to decide you want to submit a period piece on Japanese architecture rather than a novel about Argentinean cattle rustling. But your publisher doesn't have to accept it.
Here's two suggestions to help keep you on track:
1. Keep a copy of the outline that you submitted to your publisher and check it every now and then to see if you're really staying to the story that you promised to deliver. For a short piece this will be basically a check-in at the end, but for a longer work or book you'll want to do this a few times to make sure you're on the same story now that you pitched back then. And if you're moving away from it...
2. Contact your editor and make sure they're okay with your new direction. Some authors want nothing to do with their publisher while they're writing: they put themselves in their cave and write until they're done. If that works for you, great, but you're taking a risk. If you check in with your editor you'll save yourself a lot of grief later in case things go sideways.
These two tips could be very important if your publisher decides they don't want to accept your manuscript. Which we'll discuss in the coming days.