Hi there? For what reason do you need a pr timeframe? actually google only know when they update pr, sometimes it is not that exact as you expected. just focus quality link/good link building to have a good pr.
Real PR is updated continuously. Toolbar PR has been updated every 3 months or so in the past. So you can see that toolbar PR <> real PR.
Google Sitemaps has a % page PR thing, which seems to have more uptodate PR than google toolbar. It only tells if your PR is None/Low/Med/High though...
I think they changed that to only a twice or three times a year, i think they dropped the seasonal thing.