Google is always updating a websites PR. I think they update the green bar around every quarter but thats an outdated figure so wouldn't worry much about it.
The next toolbar PR update should be in Apr. Google does this every three months. For your info, they are constantly updating the PR and integrates this into the SERP.
3 months for the bar is about right. But when you have PR0, you only get better ranks when the toolbar starts to show it. In my experience...
good way to know when Google's done a PR update is when the Google section is full of threads titled 'PR 5 to 0, what happened!!!!!!???'