I have no idea how long it will take. I had portions of the first half in my mind for years, but that's not the case with the second half. It's been outlined and, as long as I can keep going for long walks where I hash out my dialogue and story points (I'm sure the neighbors think I'm nuts), I can get it done in a timely manner.
(I've also made a rudimentary map of Gemenon that I've been using. I might post that sometime soon.)