Stephen King lets his books “sit” for 6 weeks after he has written them. He says he is sometimes tempted to go admire a sentence that came out just right, or go fix something that he’s been mulling over, but if he RESISTS going back and just lets it SIT, then when he returns, he can see the book well enough to reshape it and polish it.
I liken it to letting a cake cool If one cuts into their writing too soon, without the cooling off period, then the cake can sink. If you wait too long, it goes stale and it is tougher to get back into the shaping.
Once this second pass (post-cooling off) happens for me, then I usually “know” (gut feel) if the work is ready, and if I am ready, to release it. Some authors I interviewed spoke about the difficulty of letting a book go, because it finds it way into the world. Like letting a child go to kindergarten–it will find bullies, unexpected friends, things will happen to your creation that you have no control over. So there are two timings to consider. There is the time when that the work itself is ready and then there is also the question of when the author is ready, to release their work.
I think the answer is easier for non-fiction. What I found when doing research was, when the authors started repeating each other, I had come full circle. I could tell I had fished out that pond and it was time to complete the interviews. Similarly for the writing — it had a natural feeling end.
For fiction, it is not as clear for me–there is always a character who wants more attention. Negotiating with the character to see if they can “keep” until the next story is part of completion for me.