I think your "visible" pagerank updates every 3 months. Your "actual" pagerank is constantly changing. Just my take.
well as i understood it, up until about 6 months ago their was no schedule for the PR updated, it just happened when it happened but i read somewhere recently that google are to start new schedules for reindex's every 2-3 weeks, and a PR update every 3 months. Feel free to correct me if im wrong, but i read it on sitepro news i think!
Google updates every 3 months. Last update of google was last January so the next update might be on April.
Yes, "visible" PageRank is just a number update. I've seen pages with unassigned PageRank but still get top 10 results in search engines. Only a few months afterwards that the darn PageRank number will eventually appear. So please don't get too worked up on that, as long as your pages continue to appear on search engine results.
I agree that the minimum PR update is in every 3 months. But theres no exact month to predict the PR updates. Because Google is very difficult to determine the next step.
You can use Google webmaster tools to find a graph of current PR of your pages . Just gives a high, med and low indications though.
it's not fixed - and nobody knows! We only know facts from history but we cannot learn from history in this case
google mostly updates toolbar page rank 4 times in a year but in fact nobody can give the exact times . only google knows the exact time