Regardless of what rules we choose, one thing I don't see here in the comments much is the practice of setting a good example for your kids. Kids can easily identify hypocrisy. A parent can't tell their child to not get addicted to a smartphone if the parent himself is addicted to a smartphone. So as parents we set ground rules for our own conduct before even thinking about policy for the child: We should not sit there mindlessly scrolling, especially in front of the child. If we need to use the phone for some task, take it out, do the task, and show the child that you're using the smartphone as a tool to accomplish something, not for passive consumption.
If you limit your kid's screen time to some arbitrary amount that you yourself cannot even achieve, you're transparently sending the message that you're full of shit and kids know when you're full of shit.
If you limit your kid's screen time to some arbitrary amount that you yourself cannot even achieve, you're transparently sending the message that you're full of shit and kids know when you're full of shit.