There are no hard and fast rules governing the frequency that your sensor needs to be calibrated. General guidelines are suggested by your manufacturer for the system you have. In general, you should calibrate your sensors on a fixed schedule and then see how it responds. If you start by calibrating them every week, and you notice they are always right on the span and zero values, you may move the frequency back to every other week and go from there. It is really hard to give general rules because every environment is different, but certainly calibrate your sensors after any event, such as a leak in that area where the sensor has come in contact with the gas. We suggest that a new system start with a periodic calibration as suggested by the manufacturer, and then adjust it as you monitor the performance of your system from calibration to calibration.