Google's John Mueller said in the Google+ Hangout from last Friday that he'd recommend you keep your 301 redirects live and in place for at least a year after you set them up. He said "I'd aim for at least a year," when it comes to keeping your 301 redirects in place.
He said it can take 6-months to a year for Google to fully recognize a site has moved. Plus you may have people finding old links and if those no longer have redirects, they may lead to a 404 page or a parked domain, which would result in a bad user experience.
John addressed the question at the 24:11 mark in the video:
For how many months or years, we should keep 301 redirect rules OR can we remove redirect rules after some time. Will removing the rules have negative impact. Will Google again start showing old urls in search results as indexed after removing the rules.
Theoretically, a 301 redirect is a permanent redirect, so theoretically you can keep that forever. Practically, that’s probably not that reasonable, that you can keep something like that forever...
In practice, if we recognize this is a permanent redirect, we try to keep that in mind for the future as well. So if you’ve moved your site, and we have been able to recognize that your site has moved, which might take maybe a half a year, maybe a year or so, then at some point, you can take that redirect down. The thing to keep in mind there is that if there are still links to the old version of the URL, then chance are, we might show the old URL too, if you remove the 301 redirect.
If you do make a site move, then you kind of have to follow the guide we set up, which also includes kinda of reaching out to make sure everyone is updating their links to the new version so that these old links don’t end up getting lost. Otherwise, someone might click on this old link and it isn’t redirecting anymore and they may land on a 404 page or maybe on a parked domain and that is really a bad user experience.
But at the 26:18 mark, almost two minutes later, go to the point and said:
I’d aim for at least a year [for the 301 to be in place], if possible, I’d try to keep that longer as well, depending on what is reasonable in your situation.
Forum discussion at Google+.
This post was pre-written and scheduled to be posted today. Have a happy, healthy and sweet new year!