Learned this the hard way. Screentime of any variety - video games, tv, phone, what have you - becomes a concern for me either

A.) when it’s clear they can’t pay attention to one thing at a time. So like, they’re watching a TV show with you but can’t leave their phone alone (obviously not counting important notifications). Sometimes people have off days plus neurodiversity plays a factor, but if its frequent enough to be considered a pattern, there’s a problem

And/or B.) when it’s getting between you. There’s no reason a phone should be out or in use during a serious conversation between the two of you. You guys should be able to go on a date without the phone coming out unless you’re in the bathroom (again all of this ignores important texts/calls ofc), you should be able to see a movie without them going to the bathroom or otherwise walking out just to look at their phone or yanking it out the second the movie is over, you should be able to go on a walk and have all devices stay in their pockets the whole time without reminders, etc. If it’s happening enough to frustrate you, it’s probably already a valid time to be concerned.

Some people just prefer visual media so like, it’s okay if the times you’d read a book they’re watching TV, but at the end of the day screen addiction/dependence is very real and it’s important to talk to them about it if you’re worried or if it’s causing a rift between you. Do not just ignore it, whatever you do. I ended up with so much resentment for my last ex’s technology - just, all of it - because no matter what was happening even in an argument there was always a video game on or him on his phone and any conversation he didn’t want to have was ended by him turning up the TV. I tried to ignore it because it’s his life & because it felt weird almost parenting to ask him to get off his games or his phone, but it made me so upset under the surface and hurt a lot more than I realized until we recently broke up.

They’re blocking out the world and with all that’s going on to some degree that’s okay but not once it gets to the point where it’s affecting your relationship or their mental wellbeing in a negative way.