On both of my blogs, I post pretty regularly, 3 days a week. I feel that this works best since it keeps me motivated and my readers know when to expect updates. Because my blogs are informative, I think keeping them professional in some way works best.
Other blogs and bloggers may be different. For example, if you have a personal blog, you'll want to post when you have something important to mention. If it is a news blog, you will of course blog when something newsworthy occurs which could be at any time, multiple times a day. Finally, if you blog purely as a hobby or blogging is not the number one priority in your life, it makes perfect sense to blog whenever you have time or simply whenever you feel like it.