The question is meaningless without context. For example:
1. I'm editing a photo that has no client and no particular destination (magazine, private client, whatever). I fiddle around with this, I fiddle around with that, I experiment with the other. This fiddling and experimenting could go on indefinitely, precisely because the image has no client and no destination.
2. I'm editing a photo from a client who intends to publish it in his college alumni magazine, which has a deadline six days from now. This is for former students who will recognize their erstwhile classmates. It isn't a fine art pamphlet, and I face a well-defined window to submit it for publication.
In scenario 2, what tells me I'm done is a) the nature of the viewers (former students who want to recognize old friends), and b) the magazine's deadline: realistic photos within six days. In scenario 1, there are no such restrictions to tell me I'm done. There's nothing to stop me from playing with that image for literally months on end.