There is absolutely no formula that works.
I cut them on about 5 minutes, some more, some less… if a video is 45 minutes long then I would do anything up to 8 minutes, if it’s short video like 15 minutes then I would do 4 minutes.
I have tried every length and I have tried just about every portion of the video, from only talking, to only foreplay scenes, to just plain hardcore in the last 800 days. It made absolutely no difference to sales.
So there…