Google's John Mueller said yesterday in a Google Hangout on Google+ that if you use the tactic of submitting a sitemap of your old URLs to Google, so Google picks up on the redirects faster - that you shouldn't leave that sitemap up for over 6 months.
Most SEOs know that when you want Google to expedite finding your redirects, you can use a sitemap file and change the last modification date to get Google to crawl and index those URLs faster. So technically, Google will discover those redirects faster.
But John Mueller said that when you do so, don't leave the sitemap file available forever, it's best to keep it live for about 6 months he said.
Here is the transcript that starts around the 32:05 mark into the video:
You could submit them as a sitemap file. And say these URLs have all changed recently with the last modification date and we will take that last modification date and say oh well, we will try to recrawl these URLs to see what you changed there. Based on that we will probably recrawl them a little bit faster and try to reindex them a little bit faster, with that redirect in place. Meaning, they will actually drop out. So that could help for a period of time.I wouldn’t leave that sitemap file in place forever, I’d maybe remove it after half a year because it is really pointing at URLs that you don’t really want to care about any more.
Here is the video embed at the start time:
Forum discussion at Google+.
