How long should I keep 301 redirecting pages from a deprecated domain?

Question

I had an old domain that I have deprecated, but 301 redirected all results from it to my new site.

The new site is now receiving a decent amount of traffic, but I don't know if it's 301 redirected from the old site, and doing a site:[old site] still shows several thousand pages indexed.

Since all pages from the old site are 301 redirected, will they ever be removed from the index, as long as the old domain name is active?

As a rule of thumb, somewhere I got 90 days for any significant site changes.

When is it safe to burn the old domain?

Answer 1

Google says that it is best to retain control (and 301 redirecting) of your old site for at least 180 days to avoid any confusion

Source: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=83105

Answer 2

The rule of thumb is to keep it as long as possible, but one year is a minimum.

You should check your traffic sources, though, and if no traffic comes from that site during, say, two months, it'll be a good sign you can drop it.

The number of links to your old domain vs. the new one is important too, as the old ones will all be lost as soon as you drop it, which can cause your site to drop in the SERPs.