There is non fixed period or timeframe you should look at for a PR update. IF you look carfully you'll notice there is constant activity on Google and other major search engines.
I wouldn't be too worried about a PR update - long gone are the days when a page was ranking based on it's PR. Today many pages with PR=0 are ranking top 10 and amongst them are posts from blogs.
So disregard talk about PR and concentrate on creating good content and careful on the data structure as well.
Get interested in social networking and find a way of putting it to use for your users (and in turn) for yourself.
A great 2007 to all readers !
Sante