The introduction of competency-based medical education has shifted thinking from a fixed-time model to one stressing attained competencies, independent of the time needed to arrive at those competencies. In this article, the authors explore theoretical and conceptual issues related to time variability in medical training, starting with the Carroll model from the 1960s that put time in the equation of learning. They discuss mastery learning, deliberate practice, and learning curves.While such behaviorist theories apply well to structured courses and highly structured training settings, learning in the clinical workplace is not well captured in such theories or in the model that Carroll proposed. Important in clinical training are self-regulation and motivation; neurocognitive perspectives of time and learning; professional identity formation; and entrustment as an objective of training-all of which may be viewed from the perspective of the time needed to complete training. The authors conclude that, in approaching time variability, the Carroll equation is too simplistic in its application to the breadth of medical training. The equation may be expanded to include variables that determine effective workplace learning, but future work will need to examine the validity of these additional factors.