I make VERY long videos (average 40 min long) and was also concerned that they were way too long, especially as I tend to prefer short videos myself. So I set up a survey on my mailing list that asks "are my videos too long"?
To date, I have had over 3000 responses and 97.8% of the respondents have said that they love the long videos and that they look forward to them! As several of my videos have over 100,000 views I'm going with what my audience wants. In fact, many of them have commented that they don't like the shorter videos that others in my niche (building robots, working with electronics and microcontrollers) create as they don't get enough detail.
I also create a long article (4000 - 10,000 words) that accompanies every video and according to my Google Analytics they are being read, my website now gets about 8000 visitors a day.
But this may not be true in every niche, so I suggest you do what I did - ask your audience.
I also suggest that if you do long videos you structure them like they do with television shows. On TV show you seldom see the same camera angle or scene for more than a few seconds, and I do the same in my videos. I break mine up with animations, code samples, and multiple camera angles. I think it makes them more watchable.
I'd say to look at your audience retention report in your analytics and determine from there how long your videos should be. For a new youtuber i'd try to stay around the 10 - 15 minute mark at most. I am a new youtube creator myself, and I have recently tried to make my videos no longer than 20 minutes. Try your best to edit out unnecessary clips in your video to keep the videos as neatly edited as possible. Keep experimenting with video lengths and i'm sure you will find a solution.
I watch both long and short videos, my favorite channels upload something between 20 to 40 minutes per video and I really enjoy them, in my case, I try to do them as short as possible but I always end up doing longer videos.
In terms of retention, they do pretty similar, a 10 mins video tends to have the same percentage of retention of a 30 mins video, so I don't really know how to interpret that xD