how long does it take to create a crossword?

I'm curious about how long it takes for a cruciverbalist to create a 15x15 crossword puzzle.

Creating the puzzle matrix of letters is one task, and creating the clues is another. Timing each activity would be interesting. I'm sure it can differ wildly, depending on the puzzle and the person.

Tyler Hinman posts the question How long does it take you to make a crossword?, but can't supply an answer. He says "I can't really put a figure on it because I pretty much never make an entire puzzle in one sitting."

Ximenes gives us a little more information. He didn't create American style puzzles, but at least we have some numbers from him. In his essay How Long Does It Take Me?, he starts evasively by saying "It is a difficult question, for I have never composed a crossword at a sitting and seldom in less than half a dozen spasms, nor have I ever timed myself" but then goes on to be more specific:

If I am lucky and there are no setbacks, the diagram may take about two hours and only one spasm (not including the initial search): if it fights, it may take four hours or more and several spasms...
The total time for clues also varies considerably. Sometimes, especially at the beginning of a spasm and in the morning, they come quickly: sometimes, especially at the end of one and in the evening, they come sluggishly; then it pays to stop and come back fresh. I may finish a set in three hours: I may take six — not often more.

Will Shortz is much more concrete. He gives us the numbers, although he doesn't say where he got them. In answer to the question How long does it take to create a crossword puzzle? he says,

It varies according to the size of the puzzle and the difficulty of the construction. On average a daily New York Times crossword takes 4-6 hours for an experienced constructor to make. A Sunday puzzle might take 6-20 hours.

Seriously? 4-6 hours to build a 15x15 matrix and create all the clues? That is awe-inspiring!