In response to Tim Baxter's question;

Anumber of years ago I checked this very thing on a Steinway Duo-Art
upright that I had.  Using a tape recorder and an oscilloscope I was
able to determine that several of the notes played up to twenty-two
repetitions per second.  This was true in the treble range.  The
mid-range and bass were, of course, somewhat slower due to the greater
mass of the hammers and the larger pneumatics.  I seem to recall that
the bass note repeated at about sixteen repetitions per second but I
don't have the notes any more so I recalling this from memory.  I am
sure about the twenty-two for the treble end.

Hal