This is an ago old questions for painters working in the “bravura” style of painting. When is a painting completed? This is something that I struggle with on a daily bases as do painters who are/were much more accomplished then I.
For an artist coming from an illustrative, photo realism background, it is really easy to get carried away with the work and render it far to completely. The key is doing just enough the be able to send your message across with as little strokes as possible. Your goal is to make your work look fresh as if it was done effortlessly, even though it took great effort and calculation.
One old art school method we used is to take a size or two larger brush then you would normally use. Force yourself to render larger shapes first and then focus on the details. In this way you are invariably works from the outside in. Once you have your larger values established you can go in with smaller brushes to render things more precisely. Remember however to work on the section that is most important first and then work from there. Or if your work is a statement of color, put down the boldest color first them work out. This way you, from the onset have established what your work is about, and everything subsequent is just a filler.
So when can you say that the painting is finished? There really is no fixed answer. Work broader to narrower observing fat over lean principles and have fun along the way.