When making a video I think that it's prudent to get to the point and then stop. However, for a vlog, you're just talking about everyday stuff so it can go a bit longer.
3 minutes is about the best time because it means that more people are willing to watch and they can watch more vlogs in their video viewing time.

My last video was longer than 3 minutes because I needed to make a point and I couldn't really condense it down much more. But I will endeavour to keep my videos around the 3 minute mark in the future. I want to bring a lot of value to the DTube community. Also, I should add that my last video 'The Death Of Steem' is meant as satire of the panic merchants.