The balance is about everything. Studying, screens and social life. IT is not an exact verified time keeping, say 4 hours = 4 hours, it can be a bit less or a bit more.
The problem with screen is the time it takes away from everything else, including reading for pleasure, friends and girlfriend and of course schoolwork.
Removing completely has been in my experience counterproductive because DS would be sulking and not doing anything, just starring at the ceiling hating me.
I make it sound easy and simple but it is not. I had to raise my voice, and be strict.In the weekend, it is study first, screen later because studies have shown that gaming has an effect on the concentration abilities so if possible, avoid them before the study time.