"For purposes of Video inventory, an impression is considered “In-View” to a user when at least fifty percent (50%) of the pixels in the ad are in the viewable browser window for a minimum of two (2) consecutive seconds."

Google here https://support.google.com/dfp_premium/answer/4574077?hl=en says "For in-stream video ads, 50% of the ad’s pixels must be visible in the browser window for a continuous 2 seconds."