Perhaps never, is the answer I most expect here. I think that writers would work something far past its best possible state if allowed to tinker endlessly. Luckily, most of us have some kind of goal or deadline -- at least self-imposed if we are not among those lucky enough to be working towards a date imposed by someone else (an editor, a slavering public eager for the next installment, etc.) I do think that sometimes good enough must be good enough. That work, at least subjective work like creative writing, must follow a bell curve of sorts. I think you can tinker and fix and improve to a certain point, then you reach the top of that curve -- a place no writer can ever realize for themselves that they've attained -- and you begin the increasingly speedy slide down the other side. At the risk of having not reached the apex, I have called the book "done for now" and submitted to an editor for help and comment. This will be the first objective view of my work, and that's a bit nerve-wracking. Okay, a LOT nerve-wracking. There is a chance that this woman, a best-selling author herself, will say, "Seriously. Time to quit." I'm hoping not, but it could certainly happen.
So wish me luck!