At what point do you say, I want out of this disastrous book relationship?
Here are a few points to give you a bit of insight:
1. When the book relationship turns sour and you feel like you are hurting yourself when you open the manuscript. When it physically, mentally, and especially emotionally hurts to try to work on the book, it's time to quit.
2. When you have all the time in the world to dive right in but you just can't. It's like dragging a kid to the dentist. Just. Cannot. Do. It.
3. You no longer see the beauty in what you have done thus far. You can no longer say, "When I wrote the first draft, I was in heaven."
4. You find yourself asking the same stupid questions of all of your writer friends, and you already know the answers. "Should I rewrite it?" being chief among them all.
5. You do ugly things to it. You highlight words to change them into other words that mean exactly the same thing.
These things and about ten more are all good reasons to say adios to the book. Some books are not fixable. Some books were never meant to be written and read. Maybe written but not read by anyone but your mother. Maybe not even her. It's OKAY to say no to the book. But it is not okay to say no to your creative drive to write one. There will be other books. You can always tear it up and use bits and pieces in another one. Don't lose sight of the fact that you are a writer, though.