Educational assessment is used throughout the world for a range of different formative and summative purposes. Wherever an assessment is developed, whether by a teacher creating a quiz for their class, or by a testing company creating a high stakes assessment, it is necessary to decide how long the test should be. Specifically, how many questions should be included and how much time will be required to answer each of them. The aim of this article is to review some of the most relevant psychometric literature on this topic and show the range of test lengths that would be implied in practice by the various recommendations.