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CEOEngr's son is almost dead on. 20mm is 0.78". A Tech at my Lexus dealer told me the height should change by around 1". It is subtle but noticeable if you're looking closely.
also the height doesn't change in few seconds, it can take some time for the compressors to pump in the air. at times it also depends on where the car is level at. sometimes the car has to move a little bit for the sensor to trigger than adjust height