The ability of the ear to perceive a change in Swara varies from person to person, from an untrained to a trained ear, from noisy to quiet places, from whether 1 Swara is played or many Swaras are played at a time and so on.
It can actually be different even in the same person at different times or on different days.
It can be measured by the 'Musical Quotient' (Swaranka) as per the research work of Dr. Govind Ketkar of Dombivli (Dist. Thane, Maharashtra, India).
He has found that if a Swara changes by 2.5 %, a normal person can perceive the difference. And, a musician can find the difference even if the Swara changes by only 1.5 %.
He has also shown that by practicing certain Yoga-like techniques, this ability can be improved.
This ability is not measured in Hz because, at the lower end of a keyboard, 1 % of 'A' (110 Hz) is 1.1 Hz; whereas in the higher Saptak (Octave) of a keyboard, 1 % of the same note 'A' (880 Hz) is 8.8 Hz ! Therefore, the unit of this measurement is not 'Hz', but '% change from Hz of Shadja'.
Further, my own observation is that, if the 2 notes are played 'together' (at the same time), an average trained ear of a performing musician (like myself) can measure as minute a difference as 0.0594631 % !
This difference is nothing but '1 Cent' between any 2 consecutive keys on a keyboard or = 1 % of the difference in Hz between the two consecutive keys on a keyboard. (For eg., if we start from key A (220 Hz) of the keyboard, the next key B flat will be at 220 x 1.0594631 = 233.081882. The difference between these 2 keys is 233.081882 minus 220 = 13.081882 (or 100 cents). Therefore, 1 cent will be = 0.13081882. This is 0.0594631 % of 220.)
Such an extreme ability of a trained listener allows us to tune a Tanpura or any other instrument perfectly well with any Shadja on Harmonium, as we normally do before a concert.
Remember however, that '1 Cent' varies in 'Hz' from lower to higher Saptak on a keyboard.