When I change domains, how long should I leave the redirects in place?

If I get a new domain and want to 301 redirect www.old-domain.com to www.new-domain.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? Jacob, Denmark Russian subtitles provided by Mike Shakin. Have a question? Ask it in our Webmaster Help Forum: www.google.com Want your question to be answered on a video like this? Follow us on Twitter and look for an announcement when we take new questions: twitter.com More videos: www.youtube.com Webmaster Central Blog: googlewebmastercentral.blogspot.com Webmaster Central: www.google.com