Research - what there is of it - shows that humans tend to retain information better when they learn in short bursts at intervals rather in one big chunk. This approach has been a mainstay of advice for students revising for examinations for many years, in fact. You may like to reflect on the fact that when you are learning something by heart you are more successful when you spent frequent short amounts at the task, rather than approaching it in one long session.
The phenomenon of the spacing effect, as it is called, was first identified by Hermann Ebbinghaus in an 1885 book Über das Gedächtnis. Untersuchungen zur experimentellen Psychologie (Memory: A Contribution to Experimental Psychology). It is the phenomenon whereby animals (including humans) more easily remember or learn items when they are studied a few times spaced over a long time span rather than repeatedly studied in a short span of time (what is called ‘massed presentation’). In practice, the effect suggests that intense, last-minute studying or ‘cramming’ the night before an exam is not likely to be as effective for longer term retention as studying at intervals over a longer time frame. Last minute cramming can, however, be effective for shorter term retention.
In terms of second language acquisition it seems like common sense to assume that regular, short burst of practice are likely to be more successful. At the very least, long learning sessions place greater demands on concentration. But there has been research to demonstrate the beneficial effects of spaced learning. For example, Harry P. Bahrick et al (1993), whilst acknowledging the difficulty of controlling long-term studies, having studied four individuals over a nine year period, points out the benefits of frequent recycling and spacing. Without spaced repetition of vocabulary, students are more likely to forget.
In school settings the spaced learning effect suggests that timetabling should be arranged for language lessons to be as frequent as possible (so inevitably shorter). Other things being equal, six lessons of 30 minutes might seem preferable to three lessons of one hour. Shorter lessons encourage the teacher to work at pace, they allow for considerable L2 input nearly every day and ensure that students are less likely to get bored. The spaced learning effect should lead to better retention and acquisition.
Regrettably, school administrators rarely take these things into account when organising the curriculum. Given the inadequacies of many school timetables, for language teachers it strongly suggests that the scheme of work or curriculum plan should incorporate spaced repetition of language, both vocabulary and grammar. Sensible use of regular homework can also favour more recycling. So, if you are hampered by having only one or two contacts with pupils per week, you need to make best use of homework to exploit the spacing effect.
Bahrick, H.P., Bahrick, L.E., Bahrich, A.S., and Bahrich, P.E. (1993). ‘Maintenance of Foreign Language Vocabulary and the Spacing Effect.’ Psychological Science 4/5.
Ebbinghaus, H. (1885). Über das Gedächtnis. Untersuchungen zur experimentellen Psychologie. (Memory: A Contribution to Experimental Psychology. Martino Fine Books, 2011.)