Simple answer: forever.

First, just to be overly cautious, visit the site with a header tool and verify that the site is actually redirecting with the 301 response code.

For as long as links to a URL exist, Google will continue checking that URL. With a 301 redirect in place, Google should drastically reduce how often it rechecks the original URLs, but it will periodically recheck each URL that has incoming links.

__________________
The best way to learn anything, is to question everything.