The honest answer
There's no magic number, but there is a consensus: SEO usually takes months, not weeks, to show consistent results. Some gains may appear earlier, while others will only emerge in the medium and long term. Any promise of "first position in a few days" should raise a red flag, because it's either unrealistic or involves risky techniques that could harm your site. Serious SEO is a construction process, with results that grow over time.
