How do you determine when you change out the ignition wires?

Obviously one that is causing a misfire, or is damaged, cut, broken, burned, arcing, etc. calls for replacement, but if they all look good, what other criteria do you use?

I checked resistance on mine and got from 1450 to 2100 Ohms depending on the length, shortest to longest. Most of them are right around 1800 Ohms. The resistance seems low, but high would indicate bad.

These wires are from STI, which I read are resistorized wires. I think this could be what Rovers North sells under the Proline brand but I'm not sure (I didn't buy them).

 

This is what I do, change spark plugs every 30k and plugs and wires every 60k.

 

+1 what spike said.