Obviously other than the obvious
"You've been on XYZ long enough, do something else"
As long as dcs are not totally sedantry and passive how important is it?
Eg, if you have a strict two hours a day only rule, and their homework on the pc takes 2.5 hours is that it for the day? No tv, no pc, tablet, kindle or whatever it is you might have in the house?
Does a child happily absorbed in say, Percy Jackson, for 3 hours on an ereader/tablet count as reading a book or screen time?
On here and in RL I find some people really unnecessarily snotty and judgemental about other people's children apparently having too much "screen time".
So what is too much for you and what do you class as screen time?