A:
You bring the carbonator to any store that sells them and they will sell you a replacement one, while taking back the old one - kind of like propane tanks when you return the old tank and get a new one that is already refilled.
As to how long they last, it depends on how many bottles of soda you fill up. There are 2 sizes of carbonators. The large one will make up to 130 liter bottles, but it depends on how many pumps you put into each bottle. Obviously, the more you put in, the less bottles you will get out of it.