Screen time is an interesting and challenging topic for a technology advocate. It is difficult to advocate for an activity that allows for user control and that can be directed toward nonproductive and even dangerous ends. I have tried to consider whether there are older activities that might provide guidance. Free reading might fall into this category, but I cannot say that reading has the same downside. Television viewing is probably a better example. Technology use can be like many forms of television viewing (passive and pointless), but technology offers the opportunity for more user control and greater participation.
Here is a good source from Digital Media Central that takes a position and offers links to some of the resources leading to this position.
We recommend that instead of limiting screen time, parents should instead ask themselves and their children questions about screen context (where, when and how digital media are accessed), content (what is being watched or used), and connections (whether and how relationships are facilitated or impeded). This will provide a sounder basis for family decision-making than just watching the clock.