Because propagation delay in copper is about 2/3 the speed of light, or about 200 ms, you get 5 ns/m (Newton second per metre) added to the ping, for a total round trip delay of 10 ns/m.<br />The maximum length of an Ethernet cable recommended is 100 meters, resulting in a worst-case additional ping time of around 1000 ns, which is less than a single microsecond, the finest resolution of most ping timers.<br />To sum up,  the effect is about one nanosecond per foot of cable, which is not noticeable. However, you should stick to the maximum length of 100 meters.