Steady-state visual evoked potentials (SSVEPs) have been widely applied in brain–computer interfaces, cognitive and clinical neuroscience study. Recently, 40-Hz flickering induced brainwave entrainment (BWE) was used to intervene cognition and aging-related diseases, such as Alzheimer’s disease (AD), vascular cognitive impairment, and phobic anxiety, through 1-h-long visual stimulations. The mechanism of action was believed to induce gamma oscillation in related brain regions. According to our knowledge, the amplitude of BWE at 40 Hz through a prolonged visual stimulation has not been reported. Therefore, we recorded electroencephalogram (EEG) signal induced by a high refreshing rate monitor in 50 healthy subjects. We observed that the response of BWE varied among subjects and fluctuated over time. The relationship between 40-Hz BWE and mental state was analyzed by the brainwave band ratio and signal entropies. We found that the amplitude of BWE was inverse proportionally to brainwave band α/β ratio and correlated with the sample entropy of the recorded EEG, which further connected the BWE response to the fatigue level of the subjects. We also demonstrated that a 1-min stimulation test before the 1-h-long visual stimulation was able to predict the amplitude of BWE. Finally, we showed that the measured α/β and θ/β ratios of EEG were not changed significantly before and after the 1-h-long stimulation. Our experimental results provided database for prolonged BWE, which is beneficial for the design of BWE treatment.