Split study topics fairly across a study group.
Splitting topics across a study group divides preparation but not consumption: everyone still has to sit through every teach-back. Dividing topics by members gives each person's share and the preparation hours that follow, while the session itself is the full topic count times teaching time. The saving is real but bounded — a four-person group cuts preparation to a quarter and adds a session of listening.
Workload split
Topics per person = topics / group members; preparation hours each = topics per person x preparation minutes / 60
Session length
Session minutes = topics x teach-back minutes per topic
Substantially — preparing to explain forces you to organise the material and expose gaps, which is why the person who taught a topic usually retains it best. The trade-off is shallower knowledge of the topics you only heard.
Three to five. Below three the saving is small; above five, session length grows and individual accountability weakens.