Never trust a clickbait headline like that from me. I never deliver.
Or, rather, I deliver differently and usually over more than one post.
Direct cinema — to at least some extent — is the commonality among my favorite feature documentaries. Let me watch a good story about people. Do that, and I’ll stick with you for however long.
Documentary films about issues are certainly important. I’ve made this kind of film myself. But I find that when the protagonist is the issue itself, these films can begin to drag after about 45 minutes.
Show, don’t tell.
If you can show people in human situations — e.g. overcoming adversity, achieving goals, mastering a skill, fighting the good fight — (instead of just talking about doing these things) then the length of your film doesn’t matter very much. As long as you’re otherwise doing a good job of filmmaking 😉