Brandon Sanderson
It really depends on the book, the length of the story, and how integral to magic is to that particular story. Some are as fast as a couple of days; some take months and months. It is also difficult to answer this question because I spend a lot of time thinking about a book before writing, and the Magic is often part of that. I will often spend years with an idea growing in the back of my mind before writing--and in those cases, the actual "outlining" may take a month, but that doesn't begin to cover the time spent on the idea.
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