Small group interventions 15 to 30 minutes long get greatest effects

In a meta-analysis examining the role of time (both frequency and duration) on the effectiveness of kindergarten interventions, Cavanaugh et al. (2004) documented the great variability of intensity of time in effective interventions. In terms of intensity of time implementation, three of the five interventions that were implemented for fewer than 15 minutes per instructional session resulted in large effect sizes, 1.0 or higher. The majority of interventions were implemented from 15 to 29 minutes (n = 17) and had moderate to large effect sizes. The authors concluded that interventions delivered in small-group formats either two to three times per week or daily for a minimum of 15 to 30 minutes produced the greatest effects.

Harn, B.A., Linan-Thompson, S., Roberts, G. (2008). Intensifying Instruction: Does Additional Instructional Time Make a Difference for the Most At-Risk First Graders? Journal of Learning Disabilities, 41 (2) p.115-125.