Time perception in everyday life deals with various intervals. Here we investigated whether an automatic duration-discrimination mechanism in audition operates even for intervals of an order of seconds, by using the mismatch negativity (MMN), an index of automatic change detection in audition. In Experiment 1, occasional decrements of the duration of a repetitive “standard” tone elicited an MMN in subjects ignoring auditory stimulation, even with the standard-stimulus durations over a second. Nevertheless, the MMN amplitude was significantly diminished with standard-stimulus durations of 800 ms and above, despite the fact that a constant deviant versus standard duration ratio was used. Complementary experiments varying the interstimulus interval (Experiment 2) and the magnitude of duration change (Experiment 3) yielded corroborating results. The present results suggest that automatic duration discrimination in audition operates even for durations of the order of seconds; yet its optimum time scale might be of the order of milliseconds.