Parental guidance about games tends towards either alarm or dismissal. The evidence supports something more specific.
What the research suggests
Total time matters less than what is displaced. Gaming that replaces sleep, physical activity or social contact is a problem; gaming that replaces television is largely neutral.
The research on games causing violence has not held up well. The research on sleep disruption and on displacement of other activities is more solid.
Content ratings
Age ratings on games are based on content and are reasonably reliable. They are also widely ignored, and a great many young children play games rated for adults.
Worth checking what a game actually contains rather than assuming from the title.
The parts to pay attention to
Spending. Games with in-game purchases are designed to encourage them, and children are the least equipped to resist that design. Remove stored payment details and require approval for purchases. Both platforms support this.
Communication with strangers. Voice and text chat with unknown adults is the genuine risk, more than any content in the game. Both platforms allow restricting this.
Sleep. Games are engaging and stopping is hard. A defined end time well before bed matters more than the total hours.
What works better than restriction
Knowing what they play. Playing it occasionally. That makes conversations about it possible and makes limits negotiable rather than arbitrary.
When to be concerned
When gaming displaces everything else, when stopping causes distress out of proportion, or when it is clearly being used to avoid something. Those warrant a conversation with a professional rather than a rule.