You can easily use the "Timing" question type on the same page your video is, on the options on the right the first one is "Enable submit after (seconds)", simply add the length of your video here in seconds and the next button will be hidden until the video's time has run. It's not perfect, the user might not hit play straight away (if you don't have autoplay enabled) but it is very simple to implement and I've found fairly reliable. If you do have auto-play enabled you can also think of having "Auto-advance after (seconds)" used to move the page forward a second or two after the video's finished running.
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