I think google update page rank every three months. Just check your page rank on PR tool because sometimes google toolbar was not accurate.
Well since the last one was late april and the rumor for the next one is july, Id say every 3-4 months.
The PR update that is happening is actually a picture of what your blog or site look like. Google take a snapshot of a web(with link in it) at that time and run the data of it. The data and info from the snapshot then been exported(at a later date) to the toolbar and various Google DataCenters(DC). The PageRank that you see at the toolbar is the data that had been exported but your actual Pr only are run various time and only google know what is your actual PR is.