There is a common belief that the more the time students spend in the classroom the more they learn; a conception which has been supported by considerable empirical research. However, most of these studies are correlational, which does not provide solid support for their conclusions. In this research work we intend to go a step further in the analysis of the influence of instruction time on students' academic achievement, using student fixed-effects to exploit the variability of instruction time within-students between-subjects for fourth grade students in 24 different countries. Our results show that instruction time does not seem to be positively associated with students' academic performance for any of the countries under analysis, even when students remain engaged with the lesson during this instruction time. Based on these results, policy interventions have been suggested.