Good question!
My SMARTER IN SECONDS videos are ideally less than 2 minutes in length (with the exception of my latest one), but I've only made 3 so far!
My other videos, I try to keep below 10 minutes because it's more satisfying to look at 3 digits on that time stamp. Also, attention issues are a thing and my channel is educational so I need all the concentration I can get before it fades into the abyss.