It takes maybe 2 hours for someone experienced to turn a desktop layout into a mobile responsive layout for all screen sizes using media queries. Longer than that usually means less experience.
I am a designer and HTML/CSS coder so I am aware of the challenges from both directions.
Sure the majority of websites on the web aren't SAAS tools, but the nuances of mobile vs desktop layout described in this article don't apply so much to such simple content pages.