How often should you blog?

I am in a quandary.  How often do I need to post something new on my blog to seem “current?”  I recently visited a colleague’s website and she hadn’t posted anything since October (it’s now January) and my reaction was “whoa…she is way out of date.”  But then I went back to review when the last time was that I wrote something new.  December.  I posted twice.  One of them however was a blatant sales pitch for my upcoming video shoot dates, not really a thoughtful, informative post that any one would want to read and comment on.  So really once.  Is that enough?  Does posting once a month justify even having a blog?  Many people tell me they want to start blogging and I always caution them:  do you have something worthwhile and interesting to say often enough to bother?  A newsletter might be better.  (and I do consistently publish a meaty monthly newsletter).  Also, there is this myth that if you blog then everyone on the web will contribute and comment on what you say and you’ll get a real conversation going on.  My experience is quite the opposite.  I do see folks reading the website and the blog ( I know this from checking my analytics)…but in the 3 years or so I’ve had this website up, I’ve had 14 legitimate comments.  14.  Not a typo.  If you have included a blog as part of your website (or have a blog instead of a website) and you have a different experience, let me know.  Also:  what do you think the right frequency is for posting?  Comments welcome.  Really.