It is important to blog about your book when you first begin to write about your book in your blog when you first start writing you book outline.
You want to start attracting an audience. Those who will follow your story. You can write about the process of writing. Your emotional challenges. The struggles you might face in knowing which direction to go with your writing.
You have a chance to reveal your personality on your blog and build up a bit of anticipation for the release of your book. This is a great opportunity for you to engage your new fans.
The will want to know about why you are writing the book, how much time you spend writing, where you get your inspiration and what made you decide to write you book on your particular topic.
There are no rules about the frequency you should write, I typically write once a week or every two weeks, it depends on the blog.
But get the blog started and start building your following.