When it comes to making changes to your website for SEO purposes most site owners want to see results immediately. The fact of the matter is making changes to your website for search engine optimization purposes is actually a detailed process.
The process starts with analyzing all of the things that need to be corrected on the site. You can't just start making changes without knowing what needs to be done overall. Do a site audit and then make a plan of action from the results of your site analysis and audit.
After you know where you're going and what changes need to be made comes the tedious process of actually making those changes. Since SEO involves so many different angles and implementation needs, realistically speaking the process you're looking at can take quite a while. Depending on how large the site is, how old site is, and the depth of changes needed it wouldn't be unreasonable to spend several months just working on the cleanup and changes you need to complete.
Also involved is waiting for the search engines to make changes to your content. Perhaps you had to change all of your keywords in your text and your images. Search engines need to start picking that up and making changes. That can take several weeks to a month or more to be completely picked up and adjusted in the SERPs.
Once the implementation of the changes is complete, you want to start adding new and fresh content. Actually, most sites continue running with new content being drafted in the proper manner even while changes are being made. And this fresh, seo inspired content is shared through the proper channels, in the proper manner.
Now you're up to several months into your process. Assuming you are sharing your updated content as often as possible you should be seeing some increases in traffic. Now it's just an ongoing process to maintain and do updates as needed, tweaking keywords and continuing to work on the content that your readers want to read and enjoy.
All in all, SEO professionals realize that changes and updates to a website cannot happen overnight. But you want to know how long does SEO take to start see results? For the most part, many seo pros agree, you're looking at 4 to 6 month average return to your SEO changes. But it should be well worth the wait.