As SEO is a form of organic marketing, you should expect to wait a little longer to see results than if you paid for PPC ads.
This also means that the traffic guarantee of an SEO company will be time sensitive. A reputable marketing agency will guarantee that your chosen targeted keywords will appear on a certain page of Google by a specified date.
Although the Google algorithm is constantly changing, a skilled SEO team is always prepared for changes to ensure the success of their clients.