tedster - 7:15 pm on Oct 9, 2012 (gmt 0)
Here's a useful quote from the Google blog article:
If you decide to update your page layout, the page layout algorithm will automatically reflect the changes as we re-crawl and process enough pages from your site to assess the changes. How long that takes will depend on several factors, including the number of pages on your site and how efficiently Googlebot can crawl the content. On a typical website, it can take several weeks for Googlebot to crawl and process enough pages to reflect layout changes on the site.