This is the question that all authors ask themselves sooner or later- is your MS perfect and ready to send to an agent/publisher? Is it your absolute best work and tightest writing?
Are all commas in the right place? Dialogue tags correctly punctuated? Adverbs kept in check? Is there any possible way to make this MS better?
I'm sorry to tell you that the actual answer is no. There is, most likely, always something that can be improved in your MS, even if you've revised it a gazillion times.
Luckily, agents and publishers have professional editors on staff that catch those glitches for you, and they all know that authors can only do so much revising of their work before their eyes start to cross.
There is a lot of gray area in the realm of "improvement" due to writing style. I have found that some people like shorter, economic sentences, while other very talented writers string together clauses as long as the day.
Some writers prefer certain words and use them often because they like the way they sound, while a reader or editor might not agree with the choice. All this to say that, even when you've hunted under every stone for ways to improve your MS, an agent/publisher/editor will still find errors.
So how do you know when to stop? At the risk of sounding corny, you'll probably just feel it. You'll just know that you're finished with it and can let it fly. You might send out some feelers to a few agent/publishers, and while you're waiting, continue to work on it and tweak a sentence or two.
It's a process, and it's fluid, and in the writing world there can be very few definites, so as a 'creator' it's your job to (for lack of a better term) go with the flow.
Keep revising and tweaking and meddling until it feels right, then decide what you're going to do with it. If you wake up in the middle of the night with a great idea or line of dialogue, go ahead and add it! Change that chapter title! Nobody will know! It's your story and you can mess with it until your heart's content.
And at the end of the day, remember: you wrote a book! Take a few days to recognize that wonderful accomplishment, because the next step is the minefield of publishing.
Grab your protective headgear.