I am now trying to organize Spanish lessons for my daughter (7 years old), and I have convinced some other parents to join, so that the kids could learn together. However we have a disagreement on how many times per week they should meet. The majority of parents think that 1 lesson a week is enough, but my gut feeling is that with 1 lesson a week they won't make any progress whatsoever, and that 2 lessons a week is the absolute minimum to proceed. As a last attempt to convince them, I'd like to back my opinion with some scientific data, and so I'm asking for your help.

Is there any research on optimal frequency of studies, in terms of progress/effort? It is rather obvious that the absolute maximum should be achieved at full immersion (100% of time dedicated to studying), but I would rather guess that if you calculate progress/effort in relative terms, you'd get some kind of a sigmoid curve. Knowing a rough shape of this sigmoid one could make some kind of 80/20 analysis, or find a point where the first derivative is maximal, or find the frequency that achieves 50% of the maximal learning rate, or make any other qualitative estimation of this kind. Did anyone my chance come over a plot / chart / table like that? Thank you!