GM calls it 20%, though my tester is putting it closer to 30%, and it’s the rear three windows only.
I’m in Florida where it’s legal and almost necessary to tint every window except the windshield, and putting a piece of Florida legal 24% next to the factory “20%”, there is a noticeable difference between the two, with GMs 20% letting in a LOT more light than the aftermarket 24%.
Oddly enough, they look about the same from the outside, though on the inside there is an obvious difference. Here’s my ‘18 Silverado with 20% factory tint in the rear, Florida legal 24% up front.
My recommendation, if you’re going to tint the front two windows, keep it up above 50% or so otherwise you’re going to hate driving at night with the windows up.