First, the cache of the robots.txt file must be refreshed (we generally cache the
contents for up to one day). You can speed up this process by
submitting your updated robots.txt to Google.
Even after finding the change, crawling and indexing is a complicated process that can
sometimes take quite some time for individual URLs, so it's impossible to give an
exact timeline. Also, keep in mind that even if your robots.txt file is disallowing
access to a URL, that URL may remain visible in search results despite that fact that
we can't crawl it. If you wish to expedite removal of the pages you've blocked from
Google,
submit a removal request.