How long does it take to write a book?

I've begun doing some research on a project that I'm eager to start writing (and may have already written the first few paragraphs—just to get a feel for the character's voice). This got me thinking about a question that I get asked a lot:

How long does it take me to write a book?

This is a tricky question because I don't always start a draft and write it straight through. For instance, I'm writing the 3rd book in the Guardians of the Weldafire Stone trilogy, but even as I do this, I've started this new, unrelated story.
When I wrote Blood Bound and Unbound (and a first draft of book 3), I'd drafted all of them together. After an extensive editing process with Blood Bound, the changes to Unbound were quick and fairly straight forward. The 3rd book—not so much—but it's taking shape to be exactly what it needs to be. In the case of book 3, even though I'd technically written a draft, I had to scrap it and start from scratch.

I haven't got a definitive time-frame to be able to say, "I began this project here, and finished there." Sometimes I have a story brewing in my thoughts for years, and when I sit to write it, I get it down quickly because I've had it all along. Other times, I stumble upon many road-blocks that slow the process. Yet, always letting the story unfold naturally, and allowing the characters to use their own voices (because believe me, they certainly do use their voice, and will often steer things away from what I might have originally had planned).

Every story is different. Every author is different. There's no "one size fits all". But, when it comes down to it, my goal is not to write a book in record time.
My goal is to write the story that needs to be told, in its truest, most natural way, that will invite readers to experience the world as the characters see it; to relate and to be inspired.

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