Last sunday on the 13th of January,Google again put up a crawl for PR and backlinks.The last time Google updated the PR and backlinks was on the last week of October which was after a gap of six months.But this time the gap was only for three months.And to be more precise am not sure what the crawl was mainly for PR???backlinks??or both??Well the reason it seems Google has changed it method of crawling.Earlier whenever there was a crawl for PR and backlinks only those links would be highlighted that added the PR value to the already existing PR of the website.What I could analys from three of my websites after the latest crawl was that all links present for the website would be cached by the spiders.The pr for one of my website did increase and so did it's PR but the backlinks for the other couple of websites that I work upon increased from 49 to a whopping 265!!! and from 11 to 170....respectively.
Various inputs are terming this crawl to be partial.Check this out.
i belive this page rank updates as soon as google crawles your site....its not like crawleing onxce for site content ansd oncxe for page rank...Generally this page rank decides as per the some points in your site like content updates, number of back links, popularity etxc...as of i know all are happenes at a time...
Page rank depends on many factor like quality backlink and content of page.
If number of backlink or content of the site change from last time
Then Google can update the page rank during the crawing.
It depends on crawler, as if your website is updated by some good content and also no. of backlinks to your site and traffic too, which increases your page rank.
Google check all these factors, and update the PR accordingly.