This is a very good question.
However, one has to be a bit more specific on what is meant by practice. There are two extremes here. In one extreme we have any piano related activity. Anna Magdalena Bach, simply by copying by hand copy after copy of hubby’s compositions could be said to be practising. Does it include things like memorising a piece from the score without ever getting near to the piano? If so, Glenn Gould and Walter Gieseking were practising for most of their waking hours. On the other extreme, we can define practice (just for the purposes of this discussion) as time spent at the piano repeating patterns of movements. If so, Glen Gould could rightly claim that he never practised.
And here is another interesting question. Most pianists practised a lot (whatever your definition) during their younger years, and much much less in their maturity. Ed mentioned Arrau practising up to 18 hours before he was twenty. Yet in his sixties, he never practised more than 3 hours a day, and advised his students to do the same.
So do they practice less because:
i. If you practice 18 hours a day for a number of years, after that you do not need that much practice. (Btu you still need to do 18 hours daily for a while).
ii. They were practising wrongly in their youth and wasting a lot of time, however, as they figured it out, they realised that you don’t need all that practice anyway, and they just wished they knew in their youth what they knew in their mature years.
Interestingly enough, you have pianists who firmly believe (i) or (ii). (By that I mean that there is no consensus).
Paderewski (when under Leschetizky): all day long.
Glenn Gould – Claimed he never practised (that is, at the piano). However did a lot of mental practice.
Claudio Arrau (mature) – 2 – 3 hours daily – took one month completely off every year.
Sviatoslav Richter – Claimed to practise a few hours a day – immediately dismissed by his wife who said he was lying and set the record straight: ten hours a day.
Mischa Dichter – 12 hours a day in his younger years. Then 4 – 6 hours daily.
Ivo Pogorelich – 5 hours a day (when possible)
Bella Davidovich – 3 hours a day (sometimes 4 or 5)
Willhelm Bachauss – minimum of 1 hour a day of scales and technical exercises.
Katharine Goodson – no more than four hours a day.
Guiomar Novaes – 3 – 4 hours a day (never practised technique outside pieces in her mature years).
Alexander Brailovsky – “I don’t practise very much, only five hours a day”.
Walter Gieseking – “I really need very little practice, as I do not forget what I have learned: my fingers don’t forget either”. (But then he did a lot of mental practice). Took two months away from the piano every year.
Sergei Prokofiev – “I do not need so much to practise. My hands do not forget” (But then, a lot of what he played was his own music).
Vladimir Ashkenazy (in his prime) – 5 – 7 hours daily
Alfred Brendel – (around 1980) – 5 hours a day (I understand that now he tries to confine his practise even more: 2 – 3 hours – but he compensates for it with a lot of mental practice)
Youri Egorov – 4 – 5 hours daily.
Zoltan Kocsis – in his younger years a lot. Then no more than 4 hours daily.
Garrick Ohlson – 3 – 4 hours daily.[Sources: James Cooke – Great pianists on piano playing; Elyse Mach – Great contemporary pianists speak for themselves; Jeffery Johnson (ed.) – Piano mastery – The Harriet Brower interviews; David Dubal – The world of the concert pianist; interviews in several magazines].