The question is, where to you stop tweaking? On this image, I could conceivably stop right here and call it good. Do I though? Should I shade more than just the hair or will it become overworked? Do I add a background?
In truth what I really need is to work more on is deciding what I want from an image before I even sketch it. I've heard rumors about these crazy things called thumbnail images that help you figure out things like composition and mood before you even get to this point. Someday I may actually attempt to use them.
That someday isn't today however. Maybe next time?