Michelle D. Miller. (2014). Minds Online : Teaching Effectively with Technology. Harvard University Press.
The spacing effect— sometimes also called distributed practice— refers to the increased payoff we get from spreading review sessions over time, rather than “massing” them in long, concentrated sessions.31 Like the testing effect, spacing is robust and holds up under a lot of different variations. There really is no magic number of study sessions or ideal length, as long as spacing is maximized. If you have, say, six hours to spend going over previously learned material, two three- hour sessions beats one big marathon, but three, four, or more shorter sessions are even better.
… Memory researchers have cited a number of mechanisms that feed into this effect, including the ability to link up information to a wider variety of cues.
31 For a review see N. J. Cepeda, H. Pashler, E. Vul, J. T. Wixted, and D. Rohrer (2006), Distributed practice in verbal recall tasks: A review and quantitative synthesis, Psychological Bulletin 132(3): 354– 380, doi:10.1037 /0033- 29 0 18.104.22.1684 .