Snopes provides no citation for the Whistler story. That prompted me to look it up:
http://en.wikipedia.org/wiki/James_Abbott_McNeill_Whistler#R...
I had no idea that this quote has such a delightful and well-documented origin. I'd only heard the story told about Picasso (and various mechanics and engineers). A great example of how these things morph over time.
The story is delightful because it pitted two great Victorian aesthetes against one another. Ruskin had said this about Whistler:
I have seen, and heard, much of Cockney impudence before now;
but never expected to hear a coxcomb ask two hundred guineas
for flinging a pot of paint in the public's face.
So Whistler sued for defamation and was examined by Ruskin's lawyer:
Holker: Did it take you much time to paint the Nocturne in Black and Gold?
How soon did you knock it off?
Whistler: Oh, I 'knock one off' possibly in a couple of days – one day
to do the work and another to finish it.
Holker: The labour of two days is that for which you ask two hundred guineas?
Whistler: No, I ask it for the knowledge I have gained in the work of a lifetime.
The insinuation in the lawyer's question ("how soon did you knock it off?") is hilarious!
Whistler, by the way, was a great wit and had a famous skirmish with Oscar Wilde:
http://quoteinvestigator.com/2013/09/05/oscar-will/
... which inspired this Monty Python classic:
http://www.youtube.com/watch?v=UxXW6tfl2Y0