Karl Ericsson, on whose work Malcolm Gladwell based much of his popular Outliers, has expressed on a number of occasions that the optimal amount of practice time a day is around 3-5 hours, depending on domain.
There is a great deal of research in the expertise literature, but Ericsson, Krampe, Tesch-Römer (1993) is a seminal (if not uncontroversial) paper in the field which provides a very solid introduction. They reported evidence for the idea that practicing more than four to five hours a day was not associated with higher achievement within a domain. To support this claim, among others, they drew on a number of sources, including:
A number of training studies in real life have compared the efficiency of practice durations ranging from 1-8 hr per day.These studie sshow essentially no benefit from durations exceeding 4 hr per day and reduced benefits from practice
exceeding 2 hr (Welford, 1968; Woodworth & Schlosberg, 1954). Many studies of the acquisition of typing skill (Baddeley & Longman, 1978; Dvorak et al., 1936) and other perceptual-motor skills (Henshaw & Holman, 1930) indicate that the effective duration of deliberate practice may be closer to 1hr per day. Pirolli and J. R. Anderson (1985) found no increased learning from doubling the number of training trials per session in their extended training study.
- Retrospective estimates from expert violin players
When the duration of all music-related activities was summed across the diary week, the average number of hours per week was 50.6, and no reliable differences between the groups were found. Of the eight activities judged to be highly relevant to improvement of violin performance, only two had an average duration across all three groups exceeding 5 hr per week. These two activities were practice alone (19.3 hr per week) and sleep (58.2 hr per week).
- Biographies of accomplished scientists and intellectuals
Biographies report that famous scientists such as C. Darwin, (E Darwin, 1888), Pavlov (Babkin, 1949), Hans Selye (Selye, 1964), and Skinner (Skinner, 1983) adhered to a rigid daily schedule where the first major activity of each morning
involved writing for a couple of hours. In a large questionnaire study of science and engineering faculty, Kellogg (1986) found that writing on articles occurred most frequently before lunch and that limiting writing sessions to a duration of 1-2 hr was related to higher reported productivity. [...]
In this regard, it is particularly interesting to examine the way in which famous authors allocate their time. [...] Almost without exception, they tend to schedule 3-4 hr of writing every morning and to spend the rest of the day on walking, correspondence, napping, and other less demanding activities (Cowley, 1959; Plimpton, 1977).
However, many questions remain regarding the generalizability of deliberate practice. More recent research has suggested that Ericsson's theory may not be equally adequate for all domains, and nearly irrelevant to some. For a more critical review of Ericsson's work, please see my related answer on, "Do 10.000 hours of practice make you an expert?".