This research investigates the effect of perceived evaluation duration—that is, the perceived time or speed with which one generates an evaluation—on attitude certainty. Integrating diverse findings from past research, the authors propose that perceiving either fast or slow evaluation can augment attitude certainty depending on specifiable factors. Across three studies, it is shown that when people express opinions, evaluate familiar objects, or typically trust their gut reactions, perceiving fast rather than slow evaluation generally promotes greater certainty. In contrast, when people form opinions, evaluate unfamiliar objects, or typically trust more thoughtful responses, perceiving slow rather than fast evaluation generally promotes greater certainty. Mediation analyses reveal that these effects stem from trade-offs between perceived rational thought and the perceived ease of retrieving an attitude. Implications for research on deliberative versus intuitive decision making are discussed.