You will generally want to keep your promo videos short, particularly if you want people to share your videos on social media. In a way, the audience determines the length of your video. Depending on the strength of your thumbnail, they will hit the Play button. When they are bored, they will stop watching and move onto something else. Consider keeping your promo videos shorter than two minutes. Short videos have much greater engagement than longer ones. Consider making your video 30 seconds or less.