Ask HN/PG: How often does karma average get updated?

Now that the average karma is in the top bar, how often does it get updated? I recently had a comment's score deviate significantly from my current average, but my average did not change (difference was significant enough that I think it should have affected my average by at least 0.01). Thus I'm assuming it is only updated periodically (obviously, I'm also assuming it's an average per post; please correct me if this is wrong).