The best answer I can give is this--until it's done. When you start out on the journey of writing a book, you may have an idea of when you want to finish it, but the truth is until you get it written, and rewritten you never know for sure exactly how long it will take to complete it.
The worst thing you can do is try to rush it so that it fits into some predetermined time frame. You have to give a book the time it needs to unfold and the time you need to help it do so in a way that makes for a readable story. In short, don't worry about what time you'll arrive at your destination, concentrate on the journey! If you do, you'll find the trip is a lot of fun.