Does anyone know how often Google updates Ranking for webpages. I was under the impression that this was done once a month, however, it seems slower now.
No it's not once a month. Until google re index or you can say re cache you can't have new ranking. If your competetion is very tough you'll not see much change. Google generally cache a good site every after 7-10 days. Within that time it calculates backlinks and after re visiting you it gives you new ranking.
PR is completely different. Apparently, Google does actually update PR internally every day, but the displayed value is only updated every 3-4 months or so. As for the original topic, it depends on the key phrase. The positions for some change every single day. I think others don't change as much.
To me it looks you are partly right. For them to calculate Pagerank they must actually perform calculations on their whole database of something like 8 billion webpages. They cannot be doing this every day. In any case the displayed value cannot be possibly updated daily. A bit more light anyone?