[U]sing results from our full sample, an increase in study-effort of one hour per day (an increase of approximately .67 of a standard deviation in our sample) is estimated to have the same effect on grades as a 5.21 point increase in ACT scores (an increase of 1.40 standard deviations in our sample and 1.10 standard deviations among all ACT test takers).

Paper here.

Quick quiz: This paper is based on survey data of Berea College students. What is the mean number of hours per day that they report studying? Answer below the fold.

3.4 hours per day; see Table 1 in the paper. 3.4 hours is more than I would have guessed; I suppose these are really are students not just tuitioners.