Juan Hello Heather here I hope you are well. We don't have any minimum length of video requirements set up. We recommend basing the length of video on what is going to be meaningful to your readers and the information that you are looking to send them.
The longer the video and the longer the reader watches the more ads can be shown as we can then show ads in the middle of the video as well as the start and end - this is provided that advertisers are bidding to show video ads to this reader.