What is the optimal length of a training session? When a practice session is too long, there will presumably be a point where no further significant gains can be made without a break. At what point will this be?

Update: This question was originally about the length of an initial training session, but it has since been rescoped to cover training sessions in general.
Principles of optimal practice duration from the expertise literature

The expertise literature and its discussion of deliberate practice provides substantial guidance on the question of optimal practice duration.
As Ericsson et al (2006) summarised:


  elite performers search continuously for optimal training
  activities, with the most effective duration and intensity, that will
  appropriately strain the targeted physiological system to induce
  further adaptation without causing overuse and injury.


With regards to optimal practice length, Ericsson et al (2006) stated (with key points bolded by me) that:


  Although the detailed nature of deliberate practice will differ across
  domains and as a function of attained skill, there appear to be limits
  on the daily duration of deliberate practice, and this limit seems to
  generalize across domains of expertise. Expert performers from many
  domains engage in practice without rest for only around an hour, and
  they prefer to practice early in the morning when their minds are
  fresh (Ericsson et al., 1993 ). Elite musicians (Ericsson, 2002) and
  athletes (Ericsson, 2001, 2003 c) report that the factor that limits
  their deliberate practice is primarily an inability to sustain the
  level of concentration that is necessary. Even more interestingly,
  elite performers in many diverse domains have been found to practice,
  on the average, roughly the same amount every day, including
  weekends, and the amount of practice never consistently exceeds five
  hours per day (Ericsson, 1996; Ericsson et al., 1993 ). The limit of
  four to five hours of daily deliberate practice or similarly
  demanding activities holds true for a wide range of elite performers
  in different domains, such as writing by famous authors (Cowley, 195
  9; Plimpton, 1977), as does their increased tendency to take
  recuperative naps. Furthermore, unless the daily levels of practice
  are restricted, such that subsequent rest and nighttime sleep
  allow the individuals to restore their equilibrium, individuals
  often encounter overtraining injuries and, eventually, incapacitating
  “burnout.” In some domains of sports, such as gymnastics, sprinting,
  and weight lifting, the max- imal effort necessary for representative
  performance is so great that the amount of daily deliberate practice
  is even further limited by factors constraining the duration of
  produc- tion of maximal power and strength.


Implications for the specifics of your question

Thus, based on the expertise literature,  four or five one-hour sessions of deliberate practice in a day with plenty of rest between and at night would be optimal if the aim is to maximise learning on the focal task in the shortest period of time.

In general, I wouldn't define optimal duration of practice in terms of amount of the task learnt. 

Also, in the real-world, the optimality criterion can vary between people. The expertise literature assumes that the goal is to maximise learning on the focal skill. However, in the real-world, a person may be learning a number of things, and also need to accomplish tangible outcomes at the same time.

References


Cowley, M. (Ed.) (195 9). Writers at work: The
Paris review interviews. New York: Viking.
Ericsson, K. A. (1996). The acquisition of expert performance: An introduction to some of the issues. In K. A. Ericsson (Ed.), The road to excellence: The acquisition of expert performance in the arts and sciences, sports, and games (pp. 1–5 0). Mahwah, NJ: Erlbaum.
Ericsson, K. A. (2001). The path to expert
performance: Insights from the masters on how
to improve performance by deliberate practice.
In P. Thomas (Ed.), Optimizing performance in
golf (pp. 1–5 7). Brisbane, Australia: Australian
Academic Press.
Ericsson, K. A. (2002). Attaining excellence through deliberate practice: Insights from the study of expert performance. In M. Ferrari (Ed.), The pursuit of excellence in education
(pp. 21–5 5 ). Hillsdale, NJ: Erlbaum.
Ericsson, K. A. (2003c). The development of elite performance and deliberate practice: An
update from the perspective of the expert-performance approach. In J. Starkes &amp; K. A.
Ericsson (Eds.), Expert performance in sport: Recent advances in research on sport exper-
tise (pp. 49–81). Champaign, IL: Human Kinetics.
Ericsson, K. A., Krampe, R. Th., &amp; Tesch-Romer, C. (1993 ). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100, 3 63 –406. PDF
Ericsson, K. et al. (2006). The influence of experience and deliberate
practice on the development of superior expert performance. The
Cambridge handbook of expertise and expert performance, 10(3):683-703. PDF
Plimpton, G. (Ed.) (1977). Writers at work: The Paris review. Interviews, Second Series. New York: Penguin.