I'm looking at picking up a used cat-back, how many miles before the sound on these start to change because the sound insulation gets burned?

I'm sure it's a range and depends on what type of insulation and muffler, even the type of driving done. I guess I'm just looking for common experiences. The specific exhaust I'm looking at is a greddy evo2 currently, but I'd like to know experiences with all exhausts.