In a WebmasterWorld forums post, a user asks when a 301 redirect should be put in place. Should you wait for rankings to occur on a new page or should you do so immediately?
The answer is that you don't want to wait for the new pages to rank, especially because of duplicate content concerns:
If there's a possibility of an incorrect URL causing 'duplicate' issues, Google WILL find it. MSN and Yahoo seem to stick fairly rigidly to the sitemap, while Google goes walkabout. 301 asap.
How does Google react to this, they ask?
Well, we know that sometimes it takes time to reacquire those rankings.
They will start to list some of the new URLs quite quickly, but getting most of them listed will take at least 6 months. Some might not make it.
Many of the old URLs will remain in the index for many months, maybe even a year. Your redirect will get the visitor to the correct content anyway.
Once any of the old URLs are shown as Supplemental Results you cannot control what Google will show for those. Ignore them and move on. Google will do what it has to do from then on.
Forum discussion at WebmasterWorld.