In psychophysical experiments, subjects reported whether the filled part of the Oppel-Kundt stimulus was longer than the empty one at different durations of the stimuli presentations. The experimental data yielded a smooth function indicating a gradual augmentation of the illusion strength within a relatively wide 100-1000 ms interval of the exposure durations. On the contrary, the experiments with the Müller-Lyer stimuli showed a gradual decrease of the illusion magnitude within the same interval of expositions. In the supplementary experiments, the stimuli with uniformly filled or outlined rectangles of fixed duration were used; various combinations of the rectangles with the regular sequences of filling stripes were also taken. It was demonstrated that the superposition of the stimuli did not change the illusion strength noticeably. The results obtained in the study support the suggestion that the filled interval overestimation in the Oppel-Kundt stimulus may be related to spatiotemporal integration along a continuous path of neural excitation evoked by the real and illusory contours of the filling.