Effects of SEO work generally comes in stages, starting with impacts of Bug Fixes, On-Page SEO, and Off-Page SEO.  The impacts usually break down as follows:

For on-page SEO 

Impact can be immediate, especially if the site has significant bugs or technical problems that were preventing rank for some reason.  Other on-page SEO enhancements begin to show an impact in a few weeks – especially if the click through rate has been significantly improved on your pages.

Off page SEO

This is much longer term, and involves link building and content creation.  As the overall performance of your site in search engines increases, subsequent content creation may be ranked by Google much more quickly.  This “SEO slog” can sometimes be frustrating, but is one of the best investments you can make in a business.  Usually, we will work to incorporate good off-page SEO practices into normal business operations for our clients with a bit of  SEO training – particularly in PR, Marketing and Business Development staff.

Little known fact:  Google (and maybe Bing) will sometimes test a page that has been changed significantly by artificially elevating it in the results to check the click-through-rate.  If the CTR shows promise in the higher position, it may leave it higher.