Someone just asked me how long I've been working on this book.
It was greenlit on 06 July 2015, and I signed the contract on 04 September 2015.
Since I started working on it while negotiating the contract, it's fair to say I started it on the 8th of July 2015. At least, that will be my headcanon! Why? Because today is the 10th anniversary of that start-date, and...
...today is the day I'm handing off the final draft to the development editor!
So...10 years "exactly." (With a margin of error +- a few days.)