How long a muffler lasts is dependant on a number of huge variables, including the muffler material, the driving environment, and the engine and exhaust system design.
Many if not all new vehicles have stainless steel exhaust systems. If you drive a sufficient distance to work every day for the exhaust system to heat up fully, these will typically last the life of the vehicle.....unless you're ilike me and keep your vehicle forever. Even then, if you commuteon the highway like I do, they may last forever. If you only drive a mile or two to work, and you live where there are large temperature variations, you'll probably routinely have condensation in the system and it won't last as long. Even stainless steel can rust under those conditions, typically where it's been welded.
Older vehicles have steel mufflers that'll last anywhere from 3 years to indefinitely depending on the system design and the driving environment.
New Hampshire, Short drive to work? 3-5 years.
NH, highway commute? 10-15 years, maybe more.
Guam? If you stand and listen, you can hear the metal rust.