I think we stumbled upon this approach a couple years back with piano when I deviated from the teachers proscribed study plan. She recommended longer stretches on one song or concept, switching to a different one each day. Instead, I had our son spend just a few minutes on the handful of assignments (scales, review piece, improvisation, etc.) during each daily practice. By the end of the week, his total time was about the same, but it seemed as though his overall progress increased and that -- more importantly -- his frustration diminished. (And so did mine!)
I made the switch without even considering retention... it was just clear that spending more than 5-10 minutes on a difficult passage (for example) was not worth the frustration. So for us, the "retention" was a bonus.
