I assume it does for major corporate sites, but I noticed my directory updates its Google Cache every few hours: http://www.addyoursitefreesubmit.com/ Google listing: http://www.google.com/search?source...tnG=Google+Search&aq=f&aqi=&aql=&oq=&gs_rfai=
Its depend on site update history profile and not being happened for every sites. Some big social bookmark like Digg or Twitter has being crawled and cached in every few minutes. This is not because site owner has changed the crawl speed in the Webmaster Tool. It's because the site has been refreshed with a new submission / update in every few minutes for a period of time. Googlebots do remember and set their own rate for certain site.
i am using wordpress and xml sitemap, the result: every new content cached every second it is uploaded.
Well if you notice that caching is frequently happening to your site, you should take advantage of it. One thing you can do is to have good contents so Google could get more of your contents, index them and hope you show up the SERP, even on long tail keywords.
How frequently google cache your site, depends on your site update frequency. As it is a directory, so people frequently submit their site information to the directory and this why your site gets update. So google can cache your site regularly.
It is depend on how frequently you website get new content. It is power of free listing which enables you with that power ,in addition to that you will get extra value for your premium customers.
My blog new post index in Google Search within 5 seconds after published the post.My another blog takes 7 days to index new post in Google Search. Because I post new content in one week. So, it's totally depends on frequency.