How to increase watch hours: Produce content (that you want to make and is engaging), make good thumbnails, have a title that's engaging, make sure your title and description have keywords to really tap into search engine optimization so people find it when they google or youtube serach, spread the word by utilizing social media--good # and @ tags are important, invest in decent lighting and sound equipment--your phone can serve as a good camera but if lighting or sound sucks people won't stay to watch, finally familiarize yourself with filming and editing techniques by watching other channels as well as how-to vids.
Oh and I don't know if length really plays into it. I've had 5 min videos and I've had 17-20 min videos. I don't think length plays as much a role as quality. If it sucks the viewer in, they'll stay for the duration.