That moment is different for everyone, of course, but speaking in generalities, and in general relation to a specific work, to career, to anything where you create, there is a moment when you have to stand up and walk out. Physically, metaphorically, whatever. If you don't, you end up obsessed, continually reworking the piece or body of your work, second guessing yourself, hating everything old, everything new, twisting until you're tying yourself into knots. I'm sure you've all got examples that spring to your mind.
It is, however, hard to teach that moment. Kind of impossible, actually, though you can find examples of artists who are there that allow you to say, "There's someone who can't let go of their work. Don't become like him/her."
But you can't teach the moment.
It's a little internal mechanism, and it can get fucked up pretty easy, because you're never happy with your work. Well, I'm never happy. There's something terribly unsatisfying about it once it's published. I don't mean that I hate it, because I don't much have any kind of feeling for my work. No hate, no love, nothing. It's a dead thing. It's stopped squirming inside me, stopped being this slippery thought that pushed into my mind, that stopped me from having thoughts about life, and that thought has been dissected and laid across a page. Hate and love just aren't in that. I can, however, manage a craft level interest in which I look at how a good editor has changed a few sentences to make them stronger, or a bad editor weaker, and so forth, but usually I just see all the things I would do differently. And somewhere in that gaze is the realisation that nothing I've done is exact, nothing is perfect, and never will be.
I imagine, should I listen to that voice entirely, I'd be continually rewriting, cutting, shaving, jumping, until it became a smooth and barren piece. Somewhere during the week I read that style can be considered the assembly of your imperfections and, in a way, I think there's something in that. Of course, that kind of reasoning is a dangerous thing, since it can push you down a path where you're thinking every stupid choice and mistake you do is justified under the heading of style. Likewise, you can let go of a piece too early, let into the world as a half born thing, snot danging from its nose, bones brittle, maybe nothing but liquid, and you've got this ugly thing you can't justify in print. I've had that happen. So what I'm saying is that it's no easy thing to figure.
I go on feel, myself. Sometimes it's really easy to tell when you're done, because you can't stand the sight of it, and you just want to pull out the lemon coloured Bic lighter and set it on fire. I find that's the case if it's a particularly emotional kind of creation--which sounds like incredible wank, I know, but this entire post is a bit of that. But it's also true that at times a piece will be more work, will struggle to come out right, will twist, slither, and resist your vision. Just as there is that, there are the times when it rushes out in one long stretch, clean and pure. It's with this kind that it's more difficult to figure if you're done. When you have to listen to that internal mechanism, when you've got to trust what you have. Sometimes you'll be wrong, but that's not so bad. Being wrong about letting go is better than being wrong about holding on. You hold onto a piece of work and you'll reach a point where you broke it.
And broken things reassemble badly, leaving all the cracks and glue for everyone to see.