Weeks, months, or sometimes years. It depends on what niche your website is in, what resources you have allocated for SEO and how good are the backlinks you are building.
A large and solid website could see results in a couple of weeks while a new website might have to wait months as SEO is an ongoing process. There are several factors that influence where Google ranks your website and how quickly SEO takes effect. A reputable SEO company will analyze your website and SEO (and that of your competitors) and will provide an honest assessment of your potential return from SEO and how quickly you can expect results.
Results can be fast, but they can also take time. Find an SEO agency who are honest about expected results and avoid long-term contracts. The time estimate SEO companies usually give is this: about 3 months for initial SEO results, about 6 months for noticeable SEO results, and about 12 months for impactful SEO results.
Sometimes, it can take up to a year before you start to see a major impact on your business from your SEO strategy. But once you break through that threshold, the overall impact on your business can have a very long-lasting effect.