How frequently you’ll blog largely depends on your time constraints and your readers’ thirst for information from and about you. More than once a day is too much, but you probably want to have an entry every 2-3 days and certainly no less than once a week if you’re to maintain readership.
Of course, blogging will cut into your writing time, so you want to limit the length of your blogs. That’s a good idea anyway, as most online readers aren’t interested in reading particularly long entries. Between 30 and 1,250 words is good for a good a blog entry. This entry is 212 words long.
To keep blogs from diminishing your book writing time, you may want to try penning blog entries that can be used in future tomes; this is particularly useful if you’re writing nonfiction. Though my focus is on a single book, via my blog I’m actually writing five or six other books as I change up the topic matter each day. Since blogs typically allow you to see how many people are reading each entry, you can quickly get a good sense of what topics are of the most interest to readers and so refocus your efforts on the book ideas that will have the most appeal.