It’s been said that google updates their page rank every 3 or so months. Page rank is “a link analysis algorithm, named after Larry Page, used by the Google Internet search engine that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of “measuring” its relative importance within the set” according to this wikipedia article.
Before reading this wiki article, I always assumed the “page” in page rank was referring to the actual page that was being ranked. Anyway, in layman’s term, Page Rank (aka PR) helps determine the “respect level” the actual page should be given to the hyperlinked page/document/url in relation to the other pages/documents/urls that could compete for the same keyword term it is being considered to be sent back to someone who’s searching that term.
Logically, the higher the PR, the more likely it will show up as a result in the search engine results page for a particular search phrase. Examples of PR’s of 10 are Google.com and Facebook.com most likely for “search engine” and “social network” as examples. However, these PR’s are not updated daily, but rather periodically.
The last one was done according to the same wiki article on April 2nd of this past year (2010). It’s estimating Google will be doing another page rank setting of websites the day before the New Year’s.
**Update: Looks like they pulled the “estimated PR date” since it didn’t happen on 12/31/2010. However, if you’re interested in the historical page rank update data, visit this site which recorded it.