I asked my literary agent about her clients and her answer was, "About seven to nine months." And she referred to full-time writers. Writing isn't just typing words on a page, but forming words in your head before putting them on the screen, deleting them, revising, searching for exactly the right word or phrase, and going back over everything again.
Dorothy Parker once said she wouldn't write five words without deleting seven. Her hyperbolic statement is where most of the hard work in writing comes—the rewriting.
It's an individual issue. For some writers, like me, the words flow quickly; for others, it comes out a single word at a time. Because I write fast, my time is usually about four months to complete a book. (I also have a lot of energy and enjoy working long hours.)
The first professional writer I ever met said she wrote one sentence and stayed with it until she knew she had it exactly the way she wanted it. Then she went on to the second sentence. She never went back or edited. That system worked for her and it made her a slow, slow writer. As far as I know, she never finished a book on deadline, which causes problems with her publisher, but she was excellent.
How long will it take for you to write? You have to answer that for yourself.