I think it can shift rapidly when an addiction or compulsory behaviour becomes a problem.

For example, made-up numbers, but: 1-2 hours of doom-scrolling daily is okay, as it still leaves time for 1-2 hours of schoolwork at home. 3-4 hours of such doom-scrolling, doom-scrolling between classes or during classes will have a measurable negative effect on your academic knowledge and abilities. (I don't have much information on how US schoolkids live today.)