The impact of SEM is immediate. SEO takes time. Through paid SEM ads, you can start to put your results in front of audiences with just a few clicks. As soon as you launch a campaign, your ads start showing in SERPs. At any time, you can turn ads on to increase visibility or turn them off to stop showing. Conversely, SEO is something that you acquire over time and typically over a long time. It can take months of implementing an SEO strategy before a brand begins to rank on search engines.