No matter how long your video is, you have to grab their attention in the first few seconds, and keep them interested throughout the video.
Often times, I will do a search for what I'm looking for, and open 4-6 videos in separate tabs. It it does not appear to be well done, good content, and authoritative in the first few seconds, I close that tab and move on to the next one, even if it's only 1 minute long.
I cant stand the videos that start with showing a powerpoint slide of what they are going to talk about, or someone talking in a low monotone voice. Get right to the "meat and potatoes" and keep me interested! You may have the best info in the world, but if it's a bear to watch, I'm moving on! If you can keep it informative and interesting, I'll watch a 10+ minute video every time.