Just like a person might not see a flaw in themselves that another easily sees, an artist when they are making art, becomes one with it so that they are “in” it. It is only when stepping away that objectivity arrives.
We can feel an art work is finished when there is no more space to cover or stone to carve. But that is not really it, for art is malleable . . . (excepting sculpture). Some artists like to leave “mistakes” in the work to show the process. There are indigenous artisans who purposefully leave something undone—to show humility and that only God is perfect.
For me, I have come to believe that my portraits are done when the subject is able to “speak”. If my landscapes invite the viewer to enter in their realm and abide there in wonder, perhaps then they are complete.
As I write, I feel Leonardo is correct- Art is never finished, only abandoned.