from
https://www.forte-piano-pianissimo.com/ChopintheTeacher.html (which also has the bibliography to back up the quotes):
In his preface of his piano treatise, Johan Nepomuk Hummel (1778- 1837) wrote:
Many pianists, already advanced, believe that it is necessary to play at least six to seven hours a day in order to achieve their goal; they are in error: I am able to assure them that a regular, daily, attentive study of at most three hours, is sufficient for this purpose; any practice beyond this, damps the spirits, produces a mechanical, rather than an expressive and impassioned style of playing, and is generally disadvantageous to the performer, inasmuch as when compelled to play aside this incessant exercise, if called upon to play any piece on a sudden, he cannot regain his usual powers of execution without having some days previous practice.
According to one of Chopin’s student Madame Dubois, it seems that Chopin agreed with Hummel. For Delfina, another of his students, Chopin wrote: “Once again I repeat – don’t play more than two hours a day; that is quite enough during the summer.” For von Timm Chopin recommended “not practicing too long, but to reading, looking at beautiful art works, or taking walks as periods of rest from practice.” Chopin did not believe that six or eight hour practice periods “signified diligence. He considered it mechanical, unintelligent and useless labor. He insisted upon complete concentration, alertness, and attentiveness as the utmost requirements for good practicing.”