Mufflers on a new vehicle are covered under the basic warranty on the vehicle. Once the vehicle is out of warranty it’s the responsibilty of the owner to replace the muffler if it wears out.
As far as life of a replacement muffler it depends on the brand of muffler. For example, if you were to pick up a Maramount muffler and then pick up a Walker muffler for your vehicle, the Walker muffler will be heavier than the Maramount muffler because the Walker is better constructed. So it depends on how much you spend on the muffler that dictates it’s life.
Tester