Every what period does Google update their PR?

There is non fixed period or timeframe you should look at for a PR update. IF you look carfully you'll notice there is constant activity on Google and other major search engines. 

I wouldn't be too worried about a PR update - long gone are the days when a page was ranking based on  it's PR. Today many pages with PR=0 are ranking top 10 and amongst them are posts from blogs.

So disregard talk about PR and concentrate on creating good content and careful on the data structure as well. 

Get interested in social networking and find a way of putting it to use for your users (and in turn) for yourself.

A great 2007 to all readers !

Sante 

Matt Cutts, who works at Google, has posted several items pertaining to PageRank updates in his personal (non-official, but still informative) blog.  He indicates that Google updates its PR constantly, but only exports it to the Google Toolbar every few months:

http://www.mattcutts.com/blog/more-info-on-pagerank/

http://www.mattcutts.com/blog/new-toolbar-pageranks-visible/

http://www.mattcutts.com/blog/more-weather-reporting/

http://www.mattcutts.com/blog/whats-an-update/ 

Agree with Sante. Toolbar PR updates happen generally every few months, but actual PR is constant. Worrying about toolbar PR is not productive. In fact, I uninstalled the toolbar.  

What i am thinking is every time google will refresh all this.

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