You are talking about Page Rank right? Google is changing the PR behind the scene very often. But Google updates the visible PR after 2-4 months.
they export pr to toolbar between 45 days to 100 days. Last update was about 100 days while earlier update was of 45 days. Remember PR is not updated in such long days only exported to toolbar. In real pr update in 5 to 10 days. To know change in pr see the webmaster tools. If external links changes it means pr may have changed.
if you mean pr update, google is updating it every 3 months but google is updating serp with shorter times than 3 months
It just a TB PR not an actual PR of the website, still i have found from Jan.08 update. Google distributing PR each and every month.
I read somewhere that Google internally updated it's measure of your homepage's PR when the Webmaster tools say that your homepage had been accessed. Does this conincide with an update in the external links count in the Webmaster tools?
it used to be quarterly, but now it's every 2 months. Its a good indication of trust, but it really doesn't mean a real influence on serp listings. I see a lot of Pr 2 sites outranking pr 4, so don't take it too hard if you slip in pr.