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If you’ve experienced a change in ranking which you suspect may be more than a simple algorithm change, there are other things you may want to investigate as possible causes, such as a major change to your site’s content, content management system, or server architecture. For example, a site may not rank well if your server stops serving pages to Googlebot, or if you’ve changed the URLs for a large portion of your site’s pages. This article has a list of other potential reasons your site may not be doing well in search.
[edited by: tedster at 3:21 am (utc) on Jun 23, 2012]
including new layout, CMS, urls and meta title structure...
never, never, never change the urls unless you have no choice
I have done a major change to one of my sites including new layout, CMS, urls and meta title structure.
The new version was launched about one month ago.
Three, six months, maybe longer.
In my experience, if you do everything right, the worst case scenario is a 25% drop in traffic for about 3 months. However, most of the time it's more like a 10% drop for 4-6 weeks before things return to normal.