The maximum lifespan of a pH sensor is dependent on the water quality (temperature, flow, pollution, aggressive media etc.), which can vary from each water sample.
In storage a pH sensor will even age. Take storage precautions such as: Storing in a temperature controlled environment in upright position and using the KCl filled protective caps (provided with sensor).
A common sign of aging for a pH sensor is a gradual decrease in calibration slope over time.
If your sensor's calibration values are no longer inside the mathematical calculation window of the instrument, an error message will show.
If a new calibration performed with best practice methods does not improve the calibration values, the sensor requires replacement.
The mathematical window:
ideal slope - 59.2 mV/pH window - 57.0 to 60.0 mV/pH
ideal zero point - 0 mV window - ±60 mV