When is it time to replace O2 sensors?

[SVTtripleblack]

I have a little over 70k miles on my 95 cobra and I was wondering if you should replace the O2 sensors after a certain mileage interval or just wait untill you start to have problems. I am very anal about maintenance and want to keep the car in tip top shape. My car is running fine now, but I have heard to replace the o2's after 60k. What are some symptoms when they are going bad? Will you notice a large power loss and major fuel consumption if they are going bad? Any comments or advice appreciated.

 

[SVT32VDOHC]

My saying always goes "If it ain't broke, don't fix it!" I have never replaced any 5.0L oxygen sensors for maintenence. I had a 92 5.0L GT 92K tip top shape and 95% stock. It ran great, idled great, but I had a check engine light. Scanner told me EGR valve and oxygen sensor in one bank. I replaced both my light went out and stayed out. Both parts I took out looked decently clean, and were replaced with new parts from the auto parts store.

Basically what I am trying to say is I would only replace them if they go bad.