Google doesn't give out a schedule for PR updates, and in fact Matt Cutts once posted that Google considers a PR update to be a "non-event".
Tedster, I'm going to dispute that, but not in the way you were referring. When part of google's pagerank update results in sites that are gaming the system getting pagerank 7 and 8, and then turning around and selling pagerank to the highest bidder (often blatantly), this is most certainly an "event". What is worse is when these sites keep their pagerank intact for an entire cycle, and make thousands of dollars which they then invest in gaming system for the next pagerank update. This has been happening consistently for years, and there are lots of people who made reports about the scamming, and nothing was done for the entire 3 months. I am aware of several sites right now that have been reported and there is no way anyone in their right mind would say anything other than that they were gaming the system.
It is a big event for many. I can remember back in 2001 being the first to report a pagerank update, and the topic I started making to the WebmasterWorld homepage. I was excited. :) Those were the days.