Google updates the PR around 3 or 4 times a year. That is the Toolbar PR, the one seen by us in the Google Toolbar. It is only an approximation, it doesn't show an accurate, real time ranking. The actual PR, at any time, is known only by Google, so I don't think there is a reason to focus on PR that much (unless you want to sell your website or exchange some links). I have seen many low PR sites rank better than some sites with high PR, so in my opinion, concentrating on ranking well in serps is far more rewarding.