Sviatoslav Richter has some amusing comments on this, even though he did not intend to be amusing. He states in his notebooks that he never practiced more than

Sviatoslav Richter has some amusing comments on this, even though he did not intend to be amusing. He states in his notebooks that he never practiced more than three hours a day and that he kept accurate records of his practicing, which clearly demonstrated this. He then added that (of course) travel days in which you don't have access to a piano, sick days, etc. all need to be made up for. So, while he practiced more than three hours, on average it was not more than that. There then appears a footnote in the notebooks by the editor stating that even with this correction, his friends', colleagues', and the editor's own experience of Richter suggest he practiced way more than that.