Short and to the point. And don't be like so many who have a lot of vids, but have some near-eternal intro with some stupid music playing. I've seen some ten minute reviews where half of it is intro and outro. And another +1 on writing a script and staying on message. WAY too many YouTubes of people just rambling.
I think great advice has been given here. I would like to point out that it's possible, if you're good enough, to make a video that's rather long but still holds the attention of the viewer. I'm not accustomed to that. Most YT how-to or product review vids that go longer than 3 minutes are badly done and boring. However, there are exceptions. Another member here bought a mini-computer so I went looking for a YT review of it and ran across this: Minix Neo X7 Full Review - YouTube
The intro is 9 seconds long. The outro is 21 seconds long. In between is ~8 minutes of solidly-scripted, universally-informative video. 8 minutes is a LONG time to ask people to watch something but the video does everything right. When it's done the viewer is left feeling like the elapsed time should have been measured in seconds, not minutes. For people who understand computer jargon (everyone else will hate the video), this is a solid example of a high-quality review video.
I think "Keep it short" is great advice but if you know what you're doing you can choose to ignore normal best practices and still be very successful.