It's getting more and more frequent these days. Google is moving towards a more realtime approach to ranking indexes & pagerank.
That's right, 3 month is not a consistent time frame for Google updates. Whoa! Are you sure there was an update on 31 Dec '09?
hey i think every 3 months with little change in algo inlcuding PR. latest was this. http://www.searchnewz.com/latestsearch/senews/sn-4-20100104GooglesNewYearlyPageRankUpdateIsHere.html
The ideal was every 3 months but it really depends on Google on how heavy they will be updating their algorithm.
It seems that Google is updating daily!! Yesterday I had rankings on 5th page and today it dropped to 60th page!! Google is becoming more real time and taking latest news and fresh content into the search results.. But I don't think this will work...Let's wait and watch, what happens??
If you asking about PR update means, it may be after 4-5 months since its happen last December 31 only.. If you asks about update in SERP - I guess they will do when ever they wish. Some says its daily or weekly. who knows Google will do that twice in a day also. They like to keep rocking the Search Industry, so its become compulsory for them to change to make them more better. But, they will launch the Google Caffeine by this month or in next month. They told that its after holidays - you can feel the freshness in SERP because of Google Caffeine in a month or two..
I think the next PR update is either in March or April it really depends on Google since they are no longer following their 3-4 months update last time they are delayed and last December 31 they have updated which means much earlier than what is expected