The sensor life of the SV-HAT and Multigas is strange. The SV-HAT will not have a life as such, it is not consumed in operation, so the life will depend more upon how it is treated in operation. The CO2 cell will depend on the lamp life, estimated at 5 years.
The O2 sensor in the SV-Multi Sensor will have a life, as it is consumed in operation. That life will depend upon the sensor used and the environment that it is monitoring, if the O2 level is enriched or it is used at elevated pressure the life will be shorter as the sensor will be exposed to greater partial pressures of O2. If the sensor is used to breathing gases in manned spaces, I would suggest 12 month replacement to be on the safe side, however shorter duration use and regular calibration can mitigate this and allow the sensor to be used closer to its stated life form the manufacturer.
I would not recommend using a sensor where the output has dropped to below 80% of its original output in air, particularly if the endurance of the mission is more than a few days.