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Around a year ago, I added a new host provider and moved a lot of domains around. Soon afterward I saw some rankings drop on the new site. After some investigation, I found out that it was due to the old domain not being moved. The 301 was put back in place, and the rankings soon returned.
So, must one keep an old domain 301ed for eternity to a new site - or is there a time limit after which where one can sell, redevelop, etc an old domain?
If you are still getting a lot of real people looking for the site and hitting the redirect, then leave it up.
If the new site is fully indexed, and the redirect is only getting hit by spiders and search engines, then I would say its safe to take down.
The redirect should not be effecting your serps any unless there is still content on the old site that started to get reindexed when you had the 301 redirect down.
Make sure all the old content is down as well.
It seems that the 301 is mostly helping due to the backlinks for the website.
I know there's another good question in here based around do SEs ever just transfer a link's love to the new domain regardless of the 301 or do SEs always recalculate backlinks on the 301 regardless of the 301s age, etc -- just not sure of the proper wording.