In order to be approved for the Udemy marketplace, courses must have a minimum of five lectures. Out of the five, three must be video based lectures and total at least thirty minutes of video content.
Course length is advertised on the course landing page. Since not all lecture contain video, its not always obvious how the time is calculated
Time per Lecture Type
Article lectures - One minute for each 240 words
Video lectures - The duration of the video
Video &Slide Mashups lecture - The duration of the video