The best way to tell if your vinegar is ready is to taste it. If it is still too sweet, it needs to continue aging. If it is too sour, it is ready to use. You can also use a pH test strip to check the acidity of your vinegar. A pH of 3.0 or below indicates that your vinegar is ready to use. Once your vinegar is fermented, it will keep indefinitely if stored in a cool, dark place.