Most youtube let's play videos are about 15 to 20 minutes long. I decided nine month ago, mid 2016 that I am going to make 30 minutes videos in the future.
There are several reasons for that I will come to a bit later.
First of I have to clear something right away a misconception about youtube average view times.
1) It's great to have a high 'Average view duration' in the analytics tab of your channel.
2) To think that you should do 5 minutes videos because your average view time is 4 minutes and 34 seconds is wrong.
The view duration goes down if the content you produce isn't to the liking of the viewer. It doesn't matter if they watch for 10 seconds or a minute, it will pull the view duration down. If you have 30 minutes videos and you have some of these people I call “trooper” (people who watch a series, episode for episode, binging it) the view duration will go up whatever the length of your video is.
Another topic are sketches like Door Monster does it. Three to five minutes with a punch line. These aren't let's plays and the video length of those have nothing to do with the length of those.
I may be on the wrong path when I increase the length of the bite to give to the viewer. I just happen to have the time to put out 30 minutes videos per playthrough I do and I am well aware that people only watch on or at most two playthroughs of mine. At the moment that's Factorio, RimWorld and two let's plays of Stellaris, one peacefull the other isn't.
I'd argue that if someone plays only one or two similar games it might be logical to keep the videos a bit shorter. An example of this is The Loresworn Order in which case I just lack the time to watch EU4, Stellaris and CK2 and am happy if I can keep up with one.
I originally had the plan to do 40-60 minute episodes of a game I play with cutting and everything to make it actually TV like and I realised that the appeal of youtube is that it's not TV. One easily could binge or not binge or do whatever they want, which is the great success of Netflix and Amazon Prime and the others I can't remember.
On another note: I am still not sure if I should name all my episodes. At the moment all of them are numbered except the Factorio ones which have names. Usally silly ones. Does it spoiler what happens? Does it increase the click rate because people might stumble upon it?
Partial conclusion: For Factorio, 30 minutes episodes all the way all time. (I'd say the same for minecraft, you can't do anything in 20 minutes). Besides, it makes it really painfull to save scam *cough*