What is page rank, when it gets update?

What is PageRank?
PageRank is a patented method to assign a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web. Google runs on a unique combination of advanced hardware and software. The speed of google search we experience can be attributed in part to the efficiency of their search algorithm and partly to the thousands of low cost PC's they've networked together to create a superfast search engine.
The heart of their software / search algorithm is PageRank, a system for ranking web pages developed by Google founders Larry Page and Sergey Brin at Stanford University. And while they have dozens of engineers working to improve every aspect of Google on a daily basis, PageRank continues to provide the basis for all of our web search tools.

PageRank Explained
PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page's value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves "important" weigh more heavily and help to make other pages "important." Important, high-quality sites receive a higher PageRank, which Google remembers each time it conducts a search. Of course, important pages mean nothing to you if they don't match your query. So, Google combines PageRank with sophisticated text-matching techniques to find pages that are both important and relevant to your search. Google goes far beyond the number of times a term appears on a page and examines all aspects of the page's content (and the content of the pages linking to it) to determine if it's a good match for your query.

False or spoofed PageRank
Pagerank shown in Google's Toolbar can easily be manipulated by a fake pagerank number. While the Pagerank shown in the Toolbar is considered to be accurate for most sites, this value is also easily manipulated. Any low PageRank page that is redirected, via a 302 server header, to a high PageRank page causes the lower PageRank page to acquire the PageRank of the destination page. In theory a new, PR0 page with no incoming links can be redirected to STATCOUNTER Homepage - which is a PageRank 10 - and by the next PageRank update the PageRank of the new page will be upgraded to a Page Rank of 10. This is called spoofing and is a known fake pagerank.

Manipulating PageRank
For search-engine optimization purposes, some companies offer to sell high page rank links to webmasters. As links from higher-PR pages are believed to be more valuable, they tend to be more expensive. It can be an effective and viable marketing strategy to buy link advertisements on content pages of quality and relevant sites to drive traffic and increase a webmaster's link popularity. However, Google has publicly warned webmasters that if they are or were discovered to be selling links for the purpose of conferring page rank and reputation, their links will be devalued (ignored in the calculation of other pages' page ranks). The practice of buying and selling links is intensely debated across the Webmastering community. Google advises webmasters to use the nofollow HTML attribute value on sponsored links. According to Matt Cutts, Google is concerned about webmasters who try to game the system, and thereby reduce the quality of Google search results.

When Does Google Updates Pageranks?
It varies. There is no definite time interval for page rank updates. Google usually updates page rank at interval of 3 months.