If you stay on top of cleaning, and there are no signs of germs, mold and mildew, you can get away with rarely replacing the shower curtain. But if you don’t keep on top of cleaning it, and are noticing signs of mold and mildew, or general wear and tear, you should replace it.
In general, you should look to be replacing your shower curtain every year or so, depending on how grimy it is.