We're not a very structured family so I don't have limits and stuff for use. I'm sure it would be very helpful - it's just not so much my personality. I just don't care to check on everyone to make sure they haven't gone over the limit. I think they need to be responsible. When I get the feeling they aren't being responsible and their activity levels are suffering, or their chores, or I can they are just turning into lazy sloths, I just say "OFF" and I'll ban all electronics (TV, computer etc.) for the day or part of the day. They know that's what happens. So it keeps them in check most of the time ..
I find it especially challenging now that older ones are in middle school because it's the culture. I have gone down to park with my younger ones only to find all the teens who were kicked outdoors by their parents to be sitting on their bikes with their phones out. Sad. Same - as school bus goes down, my kids are next to their buds, all the heads are down staring at phones.
We just keep trying to provide opportunities for fun in other ways. I suppose that's where I direct my energy rather than strictly monitoring usage. They know they will be starting to look for part time jobs soon - if they are that bored, they can earn some money. I babysat and had a paper route (I know, what's that??? they asked) at this age.
One big thing I'm doing is getting rid of this xbox thing where they can play with their friends. Someone gave a subscription to my son as a gift, and I will be glad when it runs out. They do have all their friends on playing together so I guess it's social (?) but still - hours can go by. That's when I say "We'll drop you at the skate park" instead.
It's frustrating for sure. My mother reminds me we'd watch tv (especially TV night with all the tops shows) or talk to our friends for hours on the phone. My brother was addicted to ATARI. We're also a family of introverts, so my kids don't always want to play with each other or friends. They like days "off" so ... I go for moderation. Some days we have more electronics than the busy days. Pick our battles right?