Indefinitely. But if you're looking to use it up, Thai people use it as
A salt alternative. (Humidity in SE Asia makes crystalized salt clump) Fish sauce also has a unique depth of flavor. I add it to soups, etc.
When I need a quick fix meal, I whisk 3 eggs with a 1/2 teaspoon or so of fish sauce. I add fat to the pan -- usually a combo of unsalted butter and toasted sesame oil. Then scramble the eggs over low-medium heat. Flavorful, silky scrambled eggs.