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Hemingway rewrote the last page of A Farewell to Arms thirty-nine times. So at what point are you only successfully making a piece of writing different--instead of better? The answer--that intuitive knowledge comes with experience.
Many authors write and rewrite chapters and sometimes entire manuscripts five, ten, a gazillion times. If that’s what it takes for you to send in your best work, go for it. From my years of experience in the publishing industry, I have seen that writers who submit the cleanest work simply write one day, edit the next day, and polish when the whole thing is finished. Beyond that, you may not be making it better--only different.
This is the part where your own intuition comes into play--if you have any little niggling doubt that you can still make it better, work on it until you feel good about it. You never want to turn in the second-best version and expect your editor to rescue something you know isn’t your best work. Commit to making your writing your cleanest and best before submitting it. Whatever it takes to send in your best work, that’s what you want to strive for--and then trust your own intuition to know when enough is enough.