If I get a new domain and 301 redirect www.olddomain.com to www.newdomain.com – how long do I have to keep the redirect up before I can start using the old domain for something else? Just until it has been crawled once?
Well, this is something where search engines can change their policy over time, because we might see the web evolving, or we might see how webmasters have issues, those sorts of things.
I can tell you about my experience of moving from mattcutts.com to dullest.com and then dullest.com back to mattcutts.com. Whenever I decided to move back, I used a 301 redirect. And it took a period of several weeks, because remember, 301s happen at a page level.
What I did is – I set up the redirect such that every single page was redirecting from dullest.com to mattcutts.com so it had been a complete transition. And I really didn’t bother to check on dullest.com for a few weeks, maybe a couple months. And then when I went back and looked at Google Analytics, at that point, all of my traffic had swapped over from dullest.com to mattcutts.com.
Typically, over a period of a few weeks, or several weeks, maybe think about it like a couple months, for example, then we might be able to detect that a site has entirely moved. But if we’re getting mixed signals, like some pages return a 200, which is an OK, while other pages return a permanent or 301 redirect, then we really don’t know what to make of that. I’ve certainly seen some situations recently where a site said, “I moved from olddomain.com to newdomain.com”, but they forgot to do a subdomain. And so they were still serving 200s on the old subdomain.
It’s definitely not the case that you can assume, “oh, everything will automatically, magically work perfectly”. We do have a tool in Google Webmaster Tools where you can say, “My site has moved from here to here”. So you can do that for the 301s on each page level. But I wouldn’t just assume it only has to be crawled once. Really, Googlebot and Google need to build up enough confidence to really know that a site has fully migrated from the old site to the new site. So it can take a little while, but hopefully, after a while, we do pick up on that.
by Matt Cutts - Google's Head of Search Quality Team