Changes to your robots.txt file take effect the moment a search engine bot fetches it again. There's no propagation delay like DNS—the file either blocks or allows access instantly upon the next request. The real question is when that next fetch happens. Google generally recrawls robots.txt every few hours for established sites with regular activity. For newer or less frequently crawled domains, it might take 12–24 hours. You can speed this up by submitting your robots.txt URL directly in Google Search Console under the URL Inspection tool, which typically triggers a fetch within minutes to an hour. Bing offers similar functionality in Bing Webmaster Tools. The catch is that even after a bot reads your updated robots.txt, pages already in the crawl queue don't vanish immediately. If you just blocked a section, Google might still have those URLs queued from yesterday's crawl. They won't be crawled again, but they might remain indexed for days or weeks until Google decides to drop them. If you unblock a section, new crawls start on the next scheduled pass—which again depends on your site's crawl budget and update frequency. At Ottawa SEO, we see the fastest updates on sites that publish daily content and have strong crawl demand. A news site or active blog might see robots.txt changes reflected in crawl behavior within 2–4 hours. A static brochure site updated twice a year might wait the full 24 hours or longer. If you're troubleshooting an urgent block, always verify the live file is actually updated (typos in .htaccess rules or caching layers can interfere), then use Search Console to force a fetch. Don't assume it's instant, but don't assume it takes days either—most real-world cases fall in the 6–12 hour window for moderately active sites.