I'd say under an hour for just muffle change... It took about 2.5 - 3 hrs for the muffler quad cutout on my bumper with diffuser. I still have to take it to my body shop and get glued. And still have to do the midpipes.

But american charges $65 and hr. They said they would need about ~3Hrs for downpipes install..... and under and hr for my midpipes but didn't do them cuz of the illegal factor, scared i guess

I would assume a catback install would need....~1.5 hrs or so...