PageRank - Answers from Matt Cutts

It's a long page of Q&A regarding PageRank, but in a nutshell he is saying that PageRank is constantly calculated, but they export the ranks to the Google Toolbar every 3-4 months. So, if you see your PR suddenly jump from 4 to 5, don't expect lots of extra traffic - you would've already been getting it for up to 3-4 months.