According to Wikipedia, it depends on the insulation:
== SNIP ==
Propagation speed is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.
At data rates, transmission lines are used in copper. This is why CAT-5e has higher requirements on the twisted pairs than CAT-3 (or plain old phone line).
66% turns out to be a surprisingly consistent approximation for both copper and fiber.
According to Wikipedia, it depends on the insulation:
== SNIP ==
Propagation speed is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.
== SNIP ==
For LMR-400 (a very common cable for Ham Radio) - the Velocity Factor is about 85% according to http://www.febo.com/reference/cable_data.html
The net-net is, that there are opportunities to get closer to the speed of light if latency is really, really important.