For pianos tuned on a regular basis, it takes approximately an hour. If the piano has not been tuned for some years, or the piano has been moved into a warmer environment the ‘pitch’ may be flat of concert pitch. The piano will not match if played with a flute, clarinet, trumpet, recorder. It may take 2 hours to tune it to the correct pitch.
The frequency of a piano being tuned depends on the condition of the piano, the stability of the environment and the amount and type of use it gets.