Whether duration magnitude modulates the cognitive resources required to represent that duration in memory is a fundamental question about the cognitive architecture of interval timing. The amplitudes of the Event-Related Potential (ERP) Slow Waves (SWs) elicited when participants maintain representations of different durations (e.g., 1500 versus 3000 ms) in memory may provide an answer to this question because increased positive SWs have been associated with increased working memory demands in studies of verbal and spatial memory. Here, participants judged whether a probe stimulus (S2) was the same duration as the preceding sample stimulus (S1). SWs recorded during a 2 s delay interval between S1 and S2 presentation were significantly more positive in the 3000 ms condition than in the 1500 ms condition, particularly at frontal-central electrode locations. This result suggests that the magnitude of the duration to be remembered influences the cognitive resources required to represent that duration in working memory.