I really think it depends on the type of video you are making. I would think for a Music Video 4-6 min max, a Movie 20-30 min and for a series 10-15 min per episode. It's really about finding a balance between what you are comfortable with and the time the audience is willing to watch.
Have you ever watched Plumbella? Her videos are brilliant and hardly ever go longer than 20 minutes. I'd say 10-15 minutes is perfect. The 30+ videos tend to be full of waffling and boring every day living.
Comments
https://www.youtube.com/channel/UCQ7CQhIzLaMtgWYjOxhJf-Q
I'm also on Facebook!
https://www.facebook.com/toastplaysstuff/
You'd have to be interesting + funny as heck to make 30 min+ videos and not bore the viewers!
Good luck on your YouTube adventures
Tumblr (WIP)