Ha Ha Ha
The first draft is a raw, naked version of the story. It isn't finished until it looks pretty, and can kill you with a sharp jab from its pinky finger. How long does that take? It depends. Sometimes it's finished in the second draft, sometimes it's the sixth. The story is finished when it's as complete as it needs to be.
Then you send it to beta readers, and probably revise it again. Honestly, the book is never finished--there's always a way to improve it. The best you can do is cultivate an instinct for what works and what doesn't.
Do you cut the enigmatic assistant who works at the Apothecary, or do you elevate him/her to a larger role? I once faced this question with Alistair, and am I ever glad that I kept him! I'm confronted with it yet again in my second book, but this time the character is getting cut.
I've been chasing my tail while revising Heaven's Scar. I intended to preserve as much of the original manuscript as possible, but it isn't going to work. Neither is my indecisiveness over whether I should change the ending, or fit the revisions into the old ending. It feels wrong, therefore it is wrong.
Sometimes it's just that simple. Uhm-ing and Ah-ing don't get the words onto paper, and there comes a point in every story where you have to stop dreaming it and start whipping it into shape. You know how to edit for bad grammar and typos, and deep in your ink-pumping heart, you know how to cut out useless tangents. They're the things that will confuse and distract your reader. The shiny lumps of silver and gold you love so much that will cause your story to crash into the mountain rather than flying over it.
Every story has to end somewhere, even series. The book is finished when the room for improvement is negligible when weighed against the current incarnation.