Google seems to be doing more mini-exports these days, i.e. more than once every 3 months. My blog's toolbar PR changed yesterday and inner pages on my sites get updated quite regularly.
Google continuously updates its own pagerank database and delivers best results to searchers. but they update there toolbar results generally every 3-4 months. and even sometimes they update it quicker.
If you are considering the green bar on the toolbar, it takes 3 months nearly to get updated as the update on the PR for the toolbar takes once every 3 months mostly. And Google bots crawl through the webpages daily to provide quality links to the searches made by people. But, this crawling does not affect the PR update in a daily manner. The information collated daily is validated at the end of 3 month cycle which displays as an update in PR on the toolbar Hope this answers your question.
Google update its PR after every 3 months,last update happened on 30th dec 2008 and 2nd last on 27th sep 2008.so you can get an idea for upcoming update