There are large differences across countries in instructional time in schooling institutions. Can these differences explain some of the differences across countries in pupils' achievements in different subjects? While research in recent years provides convincing evidence about the effect of several inputs in the education production function, there is limited evidence on the effect of classroom instructional time. Such evidence is of policy relevance in many countries, and it became very concrete recently as President Barrack Obama announced the goal of extending the school week and year as a central objective in his proposed education reform for the US. In this paper, I estimate the effects of instructional time on students' academic achievement in math, science and reading. I estimate linear and non-linear instructional time effects controlling for unobserved heterogeneity of both pupils and schools. The evidence from a sample of 15 year olds from over fifty countries that participated in PISA 2006 consistently shows that instructional time has a positive and significant effect on test scores. The effect is large relative to the standard deviation of the within pupil test score distribution. I obtain similar evidence from a sample of 10 and 13 year olds in Israel. The OLS results are highly biased upward but the within student estimates are very similar across groups of developed and middle-income countries and age groups. Evidence from primary and middle schools in Israel is similar to the evidence from OECD countries. However, the estimated effect of instructional time in the sample of developing countries is much lower than the effect size in the developed countries. I also show that the productivity of instructional time is higher in countries that implemented school accountability measures, and in countries that give schools autonomy in hiring and firing teachers.