Wearable devices have been shown to effectively measure head movement during impacts in sports such as American football. The device collects kinematic signals within a predefined time window when triggered by an impact, which are then used by finite element (FE) head models to compute brain strain and strain rate. To identify the minimum sufficient time window for FE analysis, 118 video-confirmed on-field football impacts recorded by the Stanford Instrumented Mouthguard were analyzed. Simulations using truncated kinematics were compared with the original full window. Considering individual differences in brain geometry, six representative brain models were included. Larger brains required longer time windows, but a pre-trigger of 40 ms and post-trigger of 70 ms yielded strain and strain-rate calculations not significantly different from the original 200 ms window. A total duration of approximately 110 ms is recommended for accurate modeling of football impacts.