For Rich Media creatives that contain video that can be expanded to full-screen mode, the average amount of time that the video played full-screen. Only one full-screen impression is counted per impression. If a user expands to full-screen mode, then returns to standard mode, then expands to full-screen again, the full-screen time is cumulative. Time is only counted when the video is playing, not when the video is paused.
Once the creative has been displayed for more than 20 minutes, no further full-screen time is counted.