Google regular watch and update PR. There is not a fixed time and period. I recently I got better PR for my site.
IMO Google updates PR of web sites regularly 24x7 days but it shows results after a short time periods.PR updates is not a task of a single day,Google used to count back links after each cache of back links and then adds those back links for count and calculate PR of a web site or web page and after a short time it will shows results as PR increment or decrement.