Jackson Pollock the famous abstract expressionist, in response to the question, "How do you know when you're finished?" replied, "How do you know when you're finished making love?"
I would ask the same question (excluding Pollock's reply) to my art students. There are actually no right or wrong in their answers, but I believe that, a painting is finished when the artist has nothing more to give.
Just as the artist gives of his or her self to the work in progress, it is finished when the artist has nothing more to give to it.
And so, one of my art student, sick and tired of completing his work, then declared, "I have nothing more to give!!!"
When the artist has decided that the work is finished, I always my student to make it a habit to sign his artwork, even if it is just a hastily done study. As children have the tendency to write their signatures at the very edge of their work, I would ask them to indent them about a pencil's or finger's width from the edge.
The Greek artists used the expression "tetelestai" (te-tel-es-sty), an everyday language which means "it is finished". The artist would stand back at a finished painting or sculpture and say, tetelestai – it is finished, there is nothing more that can be done to make this piece of art any better. The work is complete. It was also one of Jesus' last words on the cross.
Signing the work is like putting a period at the end of the sentence, a mark that it is finished. A date or year next to the signature would easily indicate when the work was done. The signature should be legible and must distinguish the artist rather than confuse the audience. It should not be so elaborate as to be taking much attention off the main artwork. It is best to use the leftover paint used in the painting so that the colors would easily blend in the painting and would not look like an afterthought.