Through observation and experimentation, Montessori discovered the importance of a two-and-a-half to three-hour uninterrupted work period. The last hour of a lengthy work period is usually when children are most likely to choose challenging work and concentrate deeply.
Children in Montessori classrooms become absorbed in their work because they have the freedom to choose activities that interest them. In classrooms where the work period is less than two hours long, children rarely experience the deep concentration where leaps of cognitive development can take place. Children are understandably hesitant to choose challenging work if they think they won’t have time to complete it. Any interruption to the work period disrupts the child’s exploration, focus, mastery of skills, critical thinking, and problem solving.
Something to be aware of: Children who are late to school often miss class time and are less likely to choose challenging work that requires more concentration.
—Maria Montessori (author), Paul Oswald (editor), Basic Ideas of Montessori’s Educational Theory