Videos must be at least 30 seconds long to run ads, not including any logos or title cards.
The Stationary Related video player that you embed inside a post earns when a reader clicks to watch the video and a high CPM ad displays first. The longer a reader actually watches the video, with your other ads on screen as well, the more this player contributes to your overall revenue.
The Playlist video player runs on every possible pageview, running the first 30 seconds of a video in your playlist, then moving on to an ad if the reader hasn't interacted with the video player. This feature means that the length of the videos you add to your playlist doesn't matter, and you earn new video ad impressions the longer a reader spends on the page with the Playlist player open.