A lot of information about the sensor's state are available, but it doesn't mean one would want to monitor every single one of them. If you are looking for the end of the measurement sequence, you could pick your preffered source and stick with it. In typical applications as you described, you would not expect any dummy gas value. As described in the datasheet, dummy values may be inserted by the sensor only to keep a constant sampling rate and predictable device timing. It is highly recommended to use the official sensor API/drivers from Bosch Sensortec to communicate with the sensor. BME680's sensor API even provides a function called bme680_get_profile_dur(), that can be used to predict the measurement period based on the sensor's configuration. You can find the BME680 drivers here: BME680 sensor API. To benefit from the IAQ output, you will also need to use the pre-compiled BSEC library available here: BSEC Software download. If you have a look at the sample code available in this package, you will find that it makes use of bme680_get_profile_dur() after triggering a measurement, then ensures the sensor successfully returned to sleep mode before the data read-out operation.
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