"My immediate response is that no book is ever "well and truly done." They could all be made better. Every single one of them, yes, even Pride and Prejudice. There is not point at which "you shouldn't tamper with a story anymore."
Problem is that if we all took that attitude we'd all be working on the same book our entire lives and it'd only find its way into print when we carked it. Not very satisfactory. So, yes, at some point you have to let your story go. It may not be a forever letting go. It may just be letting go to send out to agents and/or editors. If it does sell it will be return to you and you'll be rewriting it again. I know some writers who continue to revise books after they've been published.
However, as a writer who's had several books under contract, deadlines are my signal to stop."
I also loved this quote, because it matches what I do:
"I stop working on a manuscript when
a) it's due, or
b) I can't stand it anymore."
She also talks about degrees of being finished.