They update when they are ready to... usually 3-4 times per year lately but not at regular intervals. Trying to guess/"predict" the next update is a waste of time (before anyone starts chiming in with bobmutch's pointless page again). Also note that it is pretty much meaningless anyway because Toolbar PR updates only bring the public display of PR up to date - the actual (internal) PR is updated on an ongoing basis so by the time they update Toolbar PR those changes are already reflected in the Google rankings.