You made good decision to create not too long videos. 10 minutes to 12 minutes are enough for watching a video as if it is longer, it will be harder for us to focus on the video.
Yeah, I think that 10-15 minutes is optimal length for such kind of videos. I doubt that you would find many people who would watch 1 hour documentary about online forum. With our fast rhytm of life not everyone has time for long videos, also, younger generation often struggle to keep attention for longer time. Some long documentary still have popularity, but it need to have lot of visual content, like documentary about history or nature.