How Much Time?

How much time do you need to spend on artwork before it is finished? When have you not done enough?
For someone like Picasso who could paint a bird on glass in a few minutes or Billy Childish who says that if a piece of work takes more than twenty minutes it really isn’t happening, the idea of spending ‘enough time’ on a piece of work is an interesting concept.
Maybe it all comes down to the vision of the artist that once it reaches that fulfilment it is done but that is a challenging problem for examination work at schools when you are dealing with children. I am just wondering if this is presented to pupils at all in their lessons. I know that it was not for me until I met one of the special teachers that you remember who do more than just teach you the subject they change you and the way that you think. My art teacher did this by asking me ‘why’. Just that simple question, not saying that I was right or wrong but he wanted me to know for myself why and the formulation of the answer made me think about why and what I was doing.
I suspect that for the pupils of art it says more about what the teacher needs to show and what boxes they need to tick than what it says about the pupil. When someone tells you that even though they have spent hours on their work it still ‘is not enough’ you need have the vision of what is required, then do it and it IS finished.