It depends on the properties of the wire, such as the diameter, the purity of the copper, type of insulation, etc. In a twisted pair wire the length the light travels is also longer than the external lenghth of the wire.
Cat5 is has about the same velocity factor as fiber: 2/3, but Cat7 is closer to 3/4. Coaxial can get even higher.
Isn't it ~2/3 c thru copper? Roughly equivalent to light-thru-glass. I'm not quite sure who is running hollow-core fiber.