I read that Dan Brown takes about two years to research a novel, and another two to write. Editing adds to the process, because it involves those first two steps. Using The DaVinci Code, and the assumption that Brown wrote it from beginning to end, I’ll explain how (with some conjecture).
Year one: Brown sees DaVinci’s painting and thinks, “that guy next to Jesus looks like a chick.” A quick Google search… she could be Mary Magdalene? “Betchya the church would have killed to keep that a secret.” Bing! Book idea. So now he researches Bible lore galore, secret codes, and the history of France and Rome. Maybe he writes chapter one.
Year two: More research! He has to go to Paris (poor guy) and the Vatican. In Italy he writes three more chapters.
Year three: Writes continually. Decides the female character must be the descendent of Jesus and Mary. Serious plot changes.
End of year four: Book done. (Party!)
Year five: Editing. Back to chapter one. Wow, it kind of sucks. It’s four years old. He has read a few books on writing historical fiction since then. He’s also seen the Louvre, and the pyramids are not where he put them. He has to apply fours years of knowledge to every old word.
So, that’s where I am now. I’m editing my American Revolution manuscript (again), but now as an author who is four years older. Better read. Wiser about the history. But, I thank Mary Magdalene that I am.