Most robots.txt changes take effect within 24 hours for the majority of websites. Google and other major crawlers cache your robots.txt file and check it periodically—typically once per day for average sites. High-traffic or frequently updated sites (news portals, major e-commerce) often see crawlers return within 1–4 hours, meaning changes propagate faster. The timing depends on your site's crawl rate and how often bots request your robots.txt. If you block a section of your site, Googlebot won't attempt to crawl those URLs on its next visit after fetching the updated file. If you unblock previously disallowed content, Google still needs to discover and crawl those pages, which can add days or weeks depending on internal linking and crawl budget. You can speed things up by using Google Search Console's robots.txt Tester tool, which fetches the live version immediately and shows exactly what Google sees. After confirming the syntax is correct, request indexing for key pages you've unblocked. For Bing, their Webmaster Tools has a similar validator. These tools don't force an instant site-wide recrawl, but they confirm your changes are live and help prioritize important URLs. Watch out for caching on your own server or CDN. If you're using Cloudfront, Cloudflare, or another CDN, purge the cache for robots.txt specifically—otherwise crawlers might fetch a stale cached version for hours or days. Also, if you accidentally block critical sections (like CSS, JS, or entire category trees), the damage is immediate once crawlers see the new file, but recovery takes longer because Google needs to recrawl and reprocess those resources. At Ottawa SEO, we always test robots.txt changes in staging first, then monitor Search Console's crawl stats and coverage reports for 48–72 hours after deployment. For large portfolio sites, we stagger changes and verify each domain individually since crawl frequency varies widely across a 500+ domain network.