There was a full PR update a few weeks ago, maybe a month now, so the PR you see today would have been showing since that last update. Google updates the visual PR about every 3 months. The real PR is held behind the scenes, so the day they update PR is the only day that the visual PR is completely correct.
Calculating PR isn’t quite as easy as stated above, but it is on the right track. Here’s a link that attempts to crack the PR code.
http://www.kenkai.com/google-pagerank-table.htm
And, Dom is right, PR is just one measure, it is the quality of links coming to your site and how you are linked that determines your placement in the SERPS.