rainborick - 4:05 pm on Jul 29, 2012 (gmt 0)
This really belongs in the Google News category, but until we get there...
Ordinarily, if the site is configured to do 301 redirects for all requests using the old domain name pointing to the same URI/file name with the new domain name, you'd expect the transition to be nearly complete after just a few weeks. A minor traffic drop is to be expected during this period, so a persistent major drop means something's wrong: either the redirects are set up improperly, the rel=canonical tags are set improperly, the changes have induced a major ranking issue, or you've coincidentally run into a change in Google's ranking methods.
The first thing to do is to check your site's status in Google's index. Does the site: operator in Google show most or all of your site's pages indexed using the new domain name and does it show a dramatically reduced number of pages indexed under the old domain name? What does Bing show for these same searches?