I simply want to have a pre-established time period to make changes where needed. The pairs could switch partners, it was completely up to them. A whole semester because they work on projects and bonding takes time. Long term CL maximizes the time students spend as active participants in the learning process. It also provides more time for students to change how they go about the process of understanding and learning mathematics.
Students develop friendships and work styles according to their mutual choices.
Rarely do group problems develop that require changing the group memberships. When this occurs I encourage them to make their own changes suggesting some options that I see according to group sizes and possible common interests. Reason for less than half a semester is to get students used to working with as many of their peers as possible. But some students got used to partners and wanted to keep the same ones through the term. The established working relationships that had developed during a whole semester.