The CO2 sensor used in co2mon.nz CO2 Monitors has a rated accuracy of ±50 ppm. The typical variations in CO2 concentration that need to be measured are hundreds of ppm in comparison.
The CO2 monitors regularly perform self-calibration and validation to maintain ongoing accuracy and confidence in their measurements. More details about monitor calibration can be found in the calibration help topic.
The CO2 monitor takes a measurement of the current CO2 level every 5-15s from a small chamber within the monitor, if the level of CO2 the monitor is exposed to changes rapdily, you will see the measurements begin to respond almost immediately with the full magnitude of any change represented within 60s as the air within the measurement chamber transitions to match the surrounding environment.
If you don't mind a spike or two in your measurement graphs, exhaling deeply towards the monitor for several breaths is an effective way to observe how quickly the monitor will respond to changes in CO2.