Knowing When to Let Go

So how do you know when you've done enough on a piece of writing? That seems to be the million dollar questions. If somene could come up with a computer program that would shout, "enough" when you're done, they'd make a mint of money. If you find yourself putting back in, the stuff you took out during the last edit, then you've probably done all you can. If you want to kill everyone in the story, then you probably need to step away from the work for a while. If you wish you'd never started it and are sure it's the worst piece of writing in the world, then it's probably time for a fresh pair of eyes and perspective. If you're convinced that this is the best thing in the world and you're going to be rich and famous, then it's probably time for an editor or another professional to intervene. The point I'm trying to make is that it's not possible to be totally dispassionate about your own work. All you can do is the best you can do. It's great if you have a professional friend, one who will tell you the truth, not what you want to hear.