Complete vs. incomplete

How much of an artwork should be left undone to make the end result interesting?? My kids' portraits were a perfect example of this experiment. 

Pastel paper comes in different shades of all colours and I enjoyed playing with different colours to get out the skin tones and yet left it undone to give the portraits a complete look. The principle of less is more works best in this medium. I do apply this to all my paintings as well keeping in mind what catches my eye first in the painting and the journey it takes thereof.