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Usually PR update happened every couple months and since the last one happened in early January 2009, I predict that the next one should be coming by mid of March. The periodic updates are the major ones, however minor adjustments keep happening in between also. If you start a blog, most likely your page rank will change from NA to 0 in a matter of a week. This does not mean that there was a PR update, it merely shows that Google knows about your blog. I have also seen pagerank drop or go up when there are no signs of any PR updates.