Hard to give an estimate, but big dinosaurs(sauropods, ceratopsians, stegosaurs, hadrosaurs, large theropods) probably had some pretty thick skin. For herbivores, it'd be for defense. Predators, protection when attacking prey or fighting each other.
Here's an analogy why thick skin would be important to herbivores. A pack of Deinonychus is attacking a Tenontosaurus. The bite and slash, but his thick skin keeps the injuries from being serious. It's like attacking a rhino with a fork. At the end of the day you got a bent fork and a pissed off rhino.
"Men like me don't start the wars. We just die in them. We've always died in them, and we always will. We don't expect any praise for it, no parades. No one knows our names."
―Alpha-98
