"Finally, deliberate practice is an effortful activity that can be sustained only for a limited time each day during extended periods without leading to exhaustion (effort constraint). To maximize gains from long-term practice, individuals must avoid exhaustion and must limit practice to an amount from which they can completely recover on a daily or weekly basis." (K. Anders Ericsson)

"A number of training studies in real life have compared the efficiency of practice durations rangingfrom 1-8 hr per day.These studiesshowessentiallyno benefit from durations exceeding 4 hr per day and reduced benefits from practice exceeding 2 hr (Welford, 1968; Woodworth & Schlosberg, 1954). Many studiesof the acquisition oftyping 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 perday." (K. Anders Ericsson)

projects.ict.usc.edu/itw/gel/EricssonDeliberatePracticePR93.pdf