Like pretty much anything in an aquarium, an air stone can last a very long time, so long s it does not get all clogged up. You should concern yourself more with the maintenance of the air stone, than worrying about it networking.
Generally you should clean your air stone every six months, along with all your aquarium equipment. All aquarium equipments should undergo a clean and maintenance twice a year, or every six months in order to make sure that it is all working well, cleaning, and that there are no clogs.
There is no lifespan of an air stone, even so, they are not very expensive and should yours stop working it should not cost you more than just a few collars to grab yourself a new one. While we say an air stone should be cleaned every 6 months, do be sure to check yourself, for some people it is weeks, for some people 3 months, for some six. It depends on your tank.