PR update happens when google runs a PR update. It used to be 6 months then it started happening every 3 months. Last update happened around the last week of July. SticKer
About every three months. The last update was around a month ago, so still some time before the next update. Who knows, this upcoming pr update may occur sooner. The last pr snapshot was old.
I think it will be sooner. Google upset a lot of people with this last update. And the end of October is to close to the end of the year when Google always does a update. End of September is my guess.
There is no definite period for PR update (but based on my own experience since I start being in SE world, PR update occurs within 3 or 4 months.)
No one knows how much time it takes but it has been noticed that google updates pagerank every three months or so...