I believe even on the lifetime guaranteed ones, it doesn’t cover the labor or other incidental parts so why bother. On my Rivieras, I never replaced them in 20-25 years. My 86 Park Avenue needed one every two years because it was driven for a couple miles and shut off. I’ve got a 95 Olds in the garage with 200K with the original muffler and pipes, so just depends. I did have to replace the cat on my 89 Riv due to rust though but not the muffler. You can usually figure one muffler and tailpipe first, then all the rest of it the second time.