Begin promoting your book when you start writing your masterpiece. Creating a website takes creativity, and building an audience for your site takes time. I know this sounds like the “horse before the cart” situation, but considering the time it takes to get noticed on the internet among the millions and millions of websites that already exist, early is better than later.
A rule of thumb is one-year from building a website to gaining a following on a book promotion website. Of course, if you are buying space on an already established website, or promoting your book on a book promotion site, you can wait for the creating your personal book launching site. My advice is the former process. Create your website, build anticipation of your book release, and build an audience to welcome your book into the market. Create anticipation, give samplings of the content to drive traffic to your website, and promote your product.