It seems that nothing’s permanent in this world, so how long is permanent when it comes to website redirects?
At Google, the system tries to reprocess all pages at least every few months. Most pages are checked much more often. However, the amount of crawling is limited in google and there are many pages that we’d like to crawl.
When a URL changes, Google systems need to see the change in the form of a redirect for at least a few times in order to record that change. To be certain that a redirect has been seen a few times, we recommend keeping the redirect in place for at least one year.
It’s also important to update all references to the old URL. That includes links the page may have within your website and critical links from outside of the website.