As of a few months ago, robots exclusions (if you are using robots.txt to block indexing) were processed the next time the page was crawled, not when the robots file was checked. The rate at which the pages were removed from the index would depend then on how quickly Google crawls your pages. For your pages to rebound from negative effects may take longer, though. Google may have to pull your full site two or three times before a minor penalty gets re-assessed and removed.

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