From
It Knows:
[1] In 1999, Google’s web index – its copy of every page on the internet – was updated once every three or four months. By 2003 parts of the index were updated once a day, and by 2007 the rate was once every few minutes. By 2009 it was no longer possible to say that the web was being crawled at such and such a speed: if Google considered there was a chance a page might be updated it engineered things such that any change on that page was reflected in its index exactly as it happened. A search for ‘hudson river’ on 15 January 2009 would have showed that a plane had crash-landed on it before it was reported by CNN.
