We take a different look at the PISA 2006 data considering time input as one of the main ingredients for scholarly achievement. Across countries there does not exist any clear relationship between total time spent studying (sum of class time, homework time and time spent in private lessons) and scholarly achievement, while more individual study time (homework time or private lessons) seems to relate negatively to scholarly achievement. On the other hand at the country level, better performing students are clearly the ones spending more time in class and doing homework. However, when considering different groups of students, this positive relationship breaks down. For instance girls, students with a migratory background, and in some countries private school students spend more time doing homework but perform worse. In order to establish a causal relationship between time input and educational output we estimate a production function for education controlling for students’ individual characteristics and different school environments. Results show that while the productivity of additional study time varies across countries, more classes and to a lesser extent more time spent doing homework have a positive effect on scholarly achievement while the effect of private lessons is negative or at most insignificant.