I was just composing a reply when DD2 appeared looking for a drink and a good night kiss.
So here is her reply (after I said we were discussing time limits on lap tops).
"I can go outside when the sun shines and go on my lap top when I'm tired and want to unwind and I'll mostly come down for tea without moaning because I can go back later".
The last point is I think the main reason why I would hate screen time limits. Generally when I say that's enough TV, SIMs, utube or in DD1's case Dr. Who, Sherlock or old fashioned book, they do stop without much grumbling.
If we had limits and rules they would constantly be either moaning about them or stretching them to breaking point. (DD2 very obviously with much grump and DD1 very quietly and deviously.)
Also because they don't feel pressured some of their computing is actually quite educational. DD2 watches loads of gymnastics and has learned lots of hair styles. That she gets DD1 to help her with. DD2 likes history and science documentaries and some of her late night TV watching really is serious BBC4.
If I limited screen time it would all be sims, pointless free games and reruns of Dr who.