pg: "There's a background process that runs every 45 seconds and picks a random loaded user to update the average of. Empirically, not more than every couple days, at best."

http://news.ycombinator.com/item?id=1875635

Also, pg posted the code to calculate karma/comment average. It doesn't include the karma of your last 5 posts or your highest ranked comment: http://news.ycombinator.com/item?id=1853688