With a new client, I can see very fast results in SERPS with on page SEO. Especially speeding up the pages, and adjusting the folds and picture size. I also make it mobile friendly and I adjust the size of the fonts larger for the mobile version of the page. Then I register the site with Web Of Trust, Norton Safeweb, and Mcafee site advisor. I make sure i put their code in my header of my landing page. Then I make sure Metatags are added to the pages and I create a XML sitemap very last. Then I go to Webmaster tools and re-ping the Google bot and 4-5 days later that page has dramatic improvements in the SERPS. All this usually takes less than 2 hours and that is almost all the initial on page SEO I personally do. Six months later I come back and reword the whole site, then re-ping again. With a fresh sitemap. But that is another story
Notice how I didn't mention doing 20 hours of initial keyword research and on page SEO for that? *wink* My keyword work begins offpage.