Google updates the toolbar when they feel like it ;-) Typically its about once per quarter. They just had an update 12/31-1/1 so I would suspect the next one will be in approximately 3 months.
there isn't any standard for this. PR gets updated very frequently but the toolbar PR gets updated approximately every 3-5 months as the fellow guys said
Google's PR cycle is DAILY. They update PR daily and incorporate that into the search ranking daily. Every 3-4 months they export a historical meaningless snapshot of that internal PR to the toolbar. By the time we see it in the toolbar, its already been factored into the search results. A simple google search will find the dates, for eg dates of google pr updates
every 3 months google updates PR officially. however PR is updated every day just use the Firefoox plugin and you can see your current PR. The last PR official update was Dec 31. 2009