This very good question came up in my Advanced Memoir class recently, and my answer is twofold:
- A piece of writing is done when you feel you have been able to get your message across. What original idea did you start out with? What feeling did you want to capture? When your readers, whether an editor or your writing group, are getting what you’ve been trying to express, then you have arrived. Getting to this point usually involves several revisions, and it also means that you have a clear mission for your piece.
- Text itself, words and sentence structure, can always be tinkered with, so in a sense, a piece of writing is never done. At some point, you have to let go, and you have to believe that your text is good enough. Just before I received the acceptance for my essay that was published in the Wall Street Journal, I looked it over again with other publications in mind and found spots where I felt I could improve the wording. Alas, it turned out it was good enough for the Walkl Street Journal as it was published the way I had submitted it.
So, the fine-tuning of editing can be an endless process, but once you as the writer feel your story or essay is delivering what you wanted it to deliver, it is done.