There is no right or wrong... as
@Travel Interesante said, it's all about your content. yes longer videos undoubtedly means more watch time, you're always going to get a few more seconds/minutes on a longer video than a shorter video, BUT, if that content is not strong enough to keep viewers coming back, there's no point. and if you end up with videos of less than 20% retention, you'll fail long term.
I have a real mix of content on my channel. 20 minute videos, 10 minutes and 4-6 minutes. Shorter ones do better in the first 7 days, longer ones do better 6 months down the road...because I'm all over the SEO/Metadata side of YT. I know I can rank certain videos VERY Well. The longer ones have a far lower Retention...BUT... they'll still always pick up views.
All I will say, my own personal little thing...
Try to aim for at least 50% retention after the first 60 seconds, make that your target, then work out what you can do on the next video to get 55%... then 60% for the video after.
I've heard some people aim for +70% after the first 60 seconds. Some claim to even achieve +75%. But I've never really seen the evidence. I personally think 60% is awesome if you have a high CTR for at least 6-8%...and obviously more than 100 views. I'm talking more like 500-1000+ views minimum. +60% after 60 seconds on just 50 views should be a given.
(Over 50% retention after 90 seconds, I usually end up with about 40% retention at the end of the Video after 500+ views)