This article discusses one of the most obvious questions in assessment design: if a test has a high stakes purpose, how long should it be? Firstly, we explore this question from a psychometric point of view starting from the (range of) minimum test reliability levels suggested in the academic literature. Then, by using published data on the typical relationship between the length, duration and reliability of exams, we develop a range of recommendations about the likely required duration of assessment. Secondly, to force deeper reflection on the results from the psychometric approach, we also compare the actual lengths of exams in England to those in other education systems around the world. Such comparisons reveal very wide variations in the amount of time young people are required to spend taking exams in different countries and at various ages. This article concludes with some reflections on how the length of exams relates to the purpose of the assessment or to how its results will be used.