Dont take my word for it, but from what I have been able to piece together they update page rank very often, because your google ranking improves with frequent updates according to this site, so I would assume that it would have to check your site quite often if this is true (
10 things you should know before submitting your site to Google » Our Picks - News, articles and reviews)
*edit: Page Rank and Google Rank are not the same thing, im not quite sure if I answered the right question. I dont know if page rank goes up due to frequent updating, but placement in results should.