I might be interested in posting a summary video on a personal website or something like that, however for YouTube I am aiming for my videos to be 10+ minutes. I want to give people quality information. However ( I think you would like this feature ) I will be posting a document/powerpoint of the content I talk about in the video in the description. That way if you want to re look at my information you don't have to re watch a 10+ minute video. This is give people a option to download all my content and keep in a folder on their computer for easy use.
10-30 mins is alright, as long as it if full of information. I would suggest fine editing and cutting the parts with "eeh", "yeah we gonna wait on this for a minute to load", etc.
Also, as you are not making a lifestyle blog, you probably shouldn't film just your face all the time. I really like videos with good presentations, as if you are studying a course somewhere. The presentation can be vary simple, but serves as a good support for your speech and helps with structuring.