Is there a guideline for how long speaker cables will last? I was adding a component recently and got a close-up look at my speaker cables that are about 20-25 years old. They're 12 gauge with a clear jacket so I could see the strands and they appeared to have lost their bright copper color. Looked dull brown to me. There wasn't any green corrosion color but I'm curious if there is an age limit. Thanks.