How much does Schema code affect SEO and site rankings  ?

Using structured data markup does not directly affect your site's ranking, so if you add this markup to your pages, and that's something that kind of helps us to better understand your page's content, but it's not something that will see any drastic changes in rankings in the search results.
using schema will help google understand your page better so that it can help google better project the website to relevant users

Hi Friends,
Google doesn't use markup for ranking purposes at this time—but rich snippets can make your web pages appear more prominently in search results, so you may see an increase in traffic.
- Source: schema.org FAQ - Webmaster Tools Help

Rich snippets can help your conversion rate because you'll have more real estate on the search engine results page, stars, a picture, etc. but Google only supports a small number of possible schema as rich snippets About rich snippets and structured data

That said, marking up your site is a good practice and no doubt search engines will continue to experiment with ways to use structured data potentially to include as a ranking factor in their algorithms.

The best place to go for best practices is Page on schema.org but also check out the Google FAQ linked to above.