When does Google update websites Pagerank?
One thing you need to remember, Google is constantly updating your web pages pagerank behind the scenes, the update actually refers to when you can publicly see the update. That means in the Google toolbar and with Pagerank lookup websites, sometimes during the update you can catch a change in some of the data centers.
Since Google doesn’t publicly announce their official Pagerank update schedule we can only go by the recorded times in the past and assume when they do updates. Currently it seems Google is updating the public Pagerank quarterly, or every 3 months. This still isn’t an exact time frame and sometimes it can go beyond 3 months before we see a public Pagerank update.
I suggest you don’t worry about Pagerank as a webmaster; instead you should spend your time and effort increasing your SERPs (Search Engine Results Position). It is not uncommon for a lower Pagerank website to rank higher in search results than a similar content site website with higher Pagerank. Consider spending your worrying on SERPs and not Pagerank.