One question artists get asked a lot is, "How do you know when a piece is finished?"
The short answer is when the artist says it is.
The longer answer is probably different for each artist. One artist I know says it's a feeling.
For me the answer is two folded, a piece is finished when:
(a) it looks as much as possible like the person / item I'm observing
(b) when I feel proud to show it to others.
Believe me that somedays I get impatient want to race to the finish line just like the hare in that famous fable. I just want to stop painting and call a piece "FINISHED".
But I can't! Not until it is as similar to the real thing as I can make it.
Rembrandt Harmenszoon van Rijn (aka Rembrandt) has a famous saying, "Choose only one master, nature." That is what I strive to do. My motivation in my art is to be truthful to what nature reveals.
If that means that a white rose petal is semi-transparent and therefore actually the color of what is behind it, well then, that is what I paint.
If that means that a person's neck reflects light from an object in the room--but which is outside of the composition--well, I still paint the reflection.
It takes concentrated focus and a willingness to let go of what I want to see so that I can see what is actually there. For me, this process of seeing truthfully takes a time. A lot of time. Certainly more time than I want it to take!
I'm sure Rembrandt, who started drawing and painting nature around the age of 13, worked much faster. Maybe someday I will too, but for now I am the tortoise in the race--slow and steady.