My son, who is now 17, spends - I would imagine, he doesn't know because he's too frightened to switch on Screen Time - maybe 10-14 hours a day glued to his phone. He's now at an age where I have no agency over him - he's an adult in 6 months and he's got to figure out his own way - but it worries me.
In his specific case I'm actually less worried than with many of his friends. He's bright, and uses the internet to figure out and watch some pretty rad stuff. It isn't all shit, and although a fair proportion of his use is Insta, it's using it as an instant messaging tool rather than a pure "social media" tool. He rarely posts, and I suspect may be different to his (female) friends in that regard.
But here are the two points that are rarely talked about that are actually the most important ones to consider:
First, if anyone was doing anything for 12 hours a day, it's probably an issue. If my kids played football for 12 hours a day, or made Lego, or cooked, I'd be concerned. It's just not healthy to be that ensconced in a thing, let alone when that thing is as hugely, highly addictive as phone use / social media. Even if that thing was the healthiest thing in the world - say someone I knew went out running for 12 hours a day - man, I'd be concerned.
Secondly, it strikes me as "out there in the world" that this is the biggest issue. To me, as a 50 year old man, it's bloody weird anyone spending this much time online, but to be outdoors, or at a bar, or a restaurant with your partner, or looking after your kids, or looking at a sunset, or on the beach - and be constantly in your phone - this is insane.
Living a life, a real life, with real relationships and real sunsets and real beaches - is an enormous privilege. Many of us are wasting it. Life is short. Leave your phone at home, and truly live it.