I agree, 5 minutes or less will keep the audience.
I have started to watch videos that start off with the history of metric threads and all i wanted to know was how to get an alternator out of a caravan. Not really, but some tubers can ramble endlessly..
I do watch a lot of car build videos, halfasskustoms being my favorite. They are 30 minutes plus.
Metalworking tutorials at make it custom, has some great content.