This paper examines student learning time as a key educational resource. It presents an overview of how different OECD countries allocate instruction time. It also develops a model to understand the effective use of allocated instruction time and examines how different OECD countries compare on this. The paper confirms the value of sufficient instruction time as a key educational resource, but the key conclusion is that what matters the most is the way in which allocated time is used. Student learning time and academic achievement seem to have complex and curvilinear relationship with diminishing returns to scale. The paper also cautions that there should be realistic expectations on how effectively students can learn throughout the school day and year. Accordingly, it suggests that instruction could be organised to better optimise times when students are better able to concentrate. Evidence on lost instruction time in different OECD countries points to areas of potential increased effectiveness within existing time allocations, for example by improving classroom management and matching instruction to better meet students' learning needs. Tables and figures are included in an annex.