I heard a wonderful quote at a workshop in SC last year: "A painting is never finished, it's abandoned."
Loved that! I could instantly relate, particularly as a colored pencil artist working in such a painfully slow medium with one of the most painfully slow techniques.
At some point in every portrait, I believe I do just decide to abandon it. Could it be tweaked a little more? Of course. Could I add a little more detail here or there? Yes. If I did, might it be improved? Possibly. Do I care? Ummm...the honest answer by the time I get to that point is "Not enough to keep working on it!"
So it might sound like I'm enouraging abandonment, but I'm really not. I have to admit that I see quite a lot of colored pencil work that in my humble opinion are only about half finished. I understand why so completely, because my own early work was abandoned too soon. It has something to do with courage - and with experience and patience.
Here is an example of an early work (mid-80's) of mine. (Click on the images to enlarge.) I have not altered the scan in any way, and the scan is very true to the original drawing. It's a self portrait - me as a child in a kimono in Japan in the 50's. At the time I drew this, I thought i'd finished it. Today, I'd consider it half way there! I imagine I thought it was finished because everything I wanted to draw was drawn. Everything had a color. There were some differences in values. What more would I need?
Let's take a look at it in a gray scale, though. Ooops! I added a little gray scale bar at the bottom so we could easily compare the values in the image with a full range of values. You can see that nothing in the piece gets even close to the end of the gray scale.. Even the darkest values don't quite make it to the middle range of value. It truly is only half finished!
If I were to paint this image today, it would probably end up closer to looking like this versioh. A higher range of values, more color saturation, smoother textures. Yes, it would take me much longer to finish than the original drawing. Yes, it would take more courage to keep pushing the values darker and darker, but the end result would be worth the extra time and effort.
I am guessing that if I knew two things then that i know now, I wouldn't have quit when the piece was half completed.
Simply isolating values makes a world of difference. A hole punched into the middle of a white piece of paper, then placed over your drawing/reference photo will make values become instantly clearer. If I'd known about that handy tool back then, I'm sure I would have kept going on this drawing.
It might sound nuts, but it's one of the best ways to check your values. Dim the ambient light, turn off your drawing light(s), stand back a few feet from your drawing, with the reference photo close to the drawing. How are you doing? Are you close in value? Or some areas too light? Are ALL areas too light? Maybe you're still just half finished! :-)