I once read a blog post by an author dithering over whether to submit something or keep working on it. She realized that this is the best version of this story I am capable of creating at this point in my writing development. I think when you've reached that point, when if you changed more things about the book you'd be changing them just for the sake of change, it's time to submit. Maybe it will turn out that the best version of the story you can create right now isn't good enough to get published. But you can sink a lot of time into a book trying to make it better, when what will improve you most as a writer is writing a whole other book.

I learned this the hard way, after years spent fiddling with an overly ambitious first novel.