Event-related brain potentials (ERPs) elicited by auditory stimuli unrelated to a current visual-cognitive task (i.e., task-irrelevant auditory probes) can be used to evaluate the level of mental workload. Towards the evaluation of workload in the shortest possible time, the present study with a multiple-stimulus paradigm (Takeda and Kimura, 2014, Int. J. Psychophysiol.) examined whether manipulating time intervals between probes could improve the temporal resolution in evaluating workload. Probes were presented in four interval conditions as a combination of two mean interval lengths [long (600 ms) vs. short (300 ms)] and two interval variabilities [variable (five levels) vs. fixed], while participants were performing a driving game at slow and fast speeds (i.e., imposing low and high workload, respectively). For each interval condition, the minimum data length required to obtain a significant difference in the amplitude of ERPs (i.e., auditory N1 and P2) between the slow and fast driving tasks was estimated. The N1 difference was significant in all four interval conditions but the required minimum data lengths to observe this difference did not greatly differ across the interval conditions (about 60-90 s). The P2 difference was significant only in the long-variable condition and the required minimum data length was about 120 s. These results suggest that, at least with a multiple-stimulus paradigm, manipulations of time intervals between probes did not greatly improve the temporal resolution in evaluating mental workload; at present, long-variable intervals would be optimal for evaluating mental workload in detail.