Blogcount has some interesting research on the blogosphere. It defines an active posting frequency as eight weeks.
Eight Weeks? This makes me feel better about my posting frequency, but eight weeks is too long.
Posting once to a blog every eight weeks and calling it active is comparable to showing six episodes of a TV show in a season and calling it a series. The medium simply promotes frequent posts.
Regardless, check out Blogcount to learn more than you ever cared to on blog specs.