Videos must be at least 10 seconds long to run ads. It's best practice not to lean too much on logos or title cards to pad content duration.
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. Individual lengths of content and ads may vary, but the total duration of ads will never exceed the duration of content.
In general, due to our ad logic, the length of the videos you add to your playlist doesn't matter. The longer a reader spends on the page with the Playlist player open, the more video ad impressions you earn.