It is recorded each watched 10 % of video length. For example, when the video length is 60 seconds, an event is sent after 0 s, 6 s, 12 s, etc. If the user views the video several times in a row, recorded values are higher than 100 %. Ie. when the user views the video 2x, the highest measured value is 200 %.
Measurements carried out using the video event. Statistics can be viewed in GA in the "Standard reporting" -> "Content" -> "Events". All videos are displayed under their YouTube code.