When first pairing your Wave with your tablet or phone, your first measurements appear within 5 minutes - 1 hour. The temperature and humidity are ready to go, but the radon sensor must undergo a 7 day calibration period, where you may observe some spikes in levels.
This calibration period is visible in the Airthings App (highlighted in yellow below):
Why does it need to acclimatize?
The Wave has a radon sensor which takes hourly recordings of the radon in your environment, and these hourly readings are combined to give you rolling averages. The Wave is designed for long term measurements, so the most reliable result is the long term average, and it must gain enough data for the results to be meaningful.
Although the calibration period is 7 days, 30 days is generally the minimum amount of time we recommend to gather a reliable amount of data. The longer you measure for, the more accurate your results will be.