How long does it take to watch a flipped lesson?

Periodically throughout the year I will poll my students on how long it takes them to watch one of my flipped videos and do the activity that goes with it. Usually I try and use a guided notes or journaling approach, but occasionally I’ll do something different. If I get consistent answers that it takes over 20 minutes then I know I need to either make the video shorter or find a different way of assessing if the video has been watched.