My pastor in LA says it typically only takes him a week or two to write a book. He waits until he has all the ideas in his head and then talks his chapters aloud to this lady who can type really fast.
Others I know take a few months, or a few years. Someone asked me how long I had been working on my book and I replied, "8 years."
With the motivation of a $20 bet, I completed my book by the 4th of July. Granted I have a ways yet to go in the editing and refining process, but I handed over a draft copy of the book to guy from down the shore before the midnight deadline.
Next Mission: Find a publisher. Praying and hoping for Harper One.