This firstly depends on the length of the written text and the amount of time users spend on each page. We suggest: you're on the safe side, if you time them to be at least 40 seconds. However, if you start your Pageflow with a background video, the video-loops should be longer, because at the beginning, the user needs some time to get his bearings and switch into full-screen mode.
Note: There is no autoplay function for mobile devices. Therefore background videos are not played. Instead you can just see a static image.