Prior eye-tracking studies involving autistic individuals have focused on total looking time or proportion of looking time to key regions of interest. These studies have not examined another important feature, the ability to sustain attention to stimuli. In particular, the ability to sustain attention to a dynamic social stimulus might reflect more advanced self-regulatory skills that may enhance engagement with and comprehension of social information. In a sample of 155 autistic children (2--8 years of age), we examined children's average look duration while they viewed a complex, dynamic stimulus containing both social and nonsocial elements. After accounting for children's age and intelligence quotient, we found that shorter average look duration was associated with increased autism spectrum disorder severity across multiple clinical measures. To calculate average look duration, we divided the length of total looking time in seconds by the total number of uninterrupted looks to the video media. Thus, the ability to sustain attention while viewing complex dynamic information could be important for comprehending dynamic social information.