$content_width for responsive UI

Question

In our "mobile first" age a theme without responsive UI is treated like from stone age. It means that a theme could have different content width for different devices. But one of requirements for theme development is to provide $content_width variable which should contain actual width of content area of a theme.

Taking in consideration said above, I have the big question what width should I set for $content_width variable? A width for mobile phones/tables/laptops/desktops?