Guidance on the calibration interval of the O2NE+ and Safe-Ox+ devices can be found in the O2NE+ & Safe-Ox+ User Manual which is available on both the O2NE+ and Safe-Ox+ product pages in the Analox Sensor Technology website.
As a rule, Analox recommends the Safe-Ox+ and O2NE+ devices are proof tested once per year (flow a challenge gs across the sensor to confirm it reads correctly). Calibration adjustment is unnecessary if your O2NE+ / Safe-Ox+ reads correctly on the proof test gas.