Intensive retreats are a wise and effective means to deepen one's understand of practice by dedicating several days to just practicing. The following two quotes illustrate the range of intensive retreats.
"Silent meditation retreats are the heart of Zen training. Weekend and week-long sesshins are meant to intensify our practice beyond what is normally possible during daily life. Characterized by silence and deep introspection, these retreats are recommended for anyone who is interested in experiencing intensive Zen training. The daily schedule of these intensives is rich. Zazen, chanting, formal meals (Oryoki), face-to-face teaching, work practice, and talks by the teachers or senior students, are all activities that, when taken up wholeheartedly, can be used as profound tools of personal transformation." Mountain Road, The Zen Practitioner's Journal, Winter 2006.
",,,The schedule consists simply of a repetition of fourteen periods of Zazen interspersed with briefer periods of walking mediation (kinhin), from four in the morning until nine in the evening. There are three meals a day and a half-hour break after each one, when everyone attends to personal needs. ... each period of zazen is fifty minutes ... there is absolutely no talking .... no greetings or socializing, and not even any of the sutra-chanting ... days of absolute silence ... ", from the book by Kosho Uchiyama, "Opening the Hand of Thought" pages 61-62.
The latter, during the 1960s in Japan, was very austere, and demanding; while the former is for sesshins in a modern monastery in upstate New York. Intensive retreats vary enormously, from days to years in length, and from very light to extremely difficult schedules.
Practice periods, generally in the spring and fall, are also very effective in deepening one's understanding. The practice period itself, perhaps three to nine weeks in duration, may include two or more periods of Zazen in the morning and/or in the evening. The period basically serves to intensify practice for a brief period, focussing toward a sesshin. The student thereby experiments with more intensive practice, and learns whether it contributes concretely to his/her daily practice.