This same information applies to many of the filters we carry, however, the life expectancy of the AP430SS filter (AP431) depends on the hardness of the water, and also the water usage.
The harder the incoming water is, the faster the phosphates inside the filter will dissolve.
The more people in the home, the more water that will be used. So a family of 6 will use up the filter faster than a family of 3, when the water hardness is the same.
To answer the original question, the filters have phosphate pellets inside of them, and if you shake the filter you can hear them rattle inside. Once the filter gets to be about 1/4 - 1/8 full (nearly empty), you should replace the filter. Anything less than that and the filter won't have enough phosphate to treat the water properly.