Speed of electricity (signal propagation?) through copper for communications delay
It's more complicated than that, the propagation speed is going to be based on both the wire and the dielectric constant of the material around the wire.  In your case that would be some plastic and air.   I'll leave the detailed explanation for someone else, but for a quick check analysis wiki says the propagation delay of cat 5 is 4.8 to 5.3 ns/meter, and the speed is 0.64 c (where c is the speed of light).  

So worst case 5.3 ns/m * 10 meters is 53ns.   Then 5.3 ns/m * 100 meters is 530ns so the difference in delay is about 477ns.