I can't give you a definite answer on the video length... there are a lot of variables to take into consideration. Mainly your audience. I would advise experimenting with the length, but really the video should be just a little shorter than it needs to be. For example, if you are going on a four-state road trip and you want to do one video about this it is reasonable to assume that you record and vlog several minutes per state. Throw in a 30 second introduction and this equates to about a ten minute video. Then go back and shave it down; edit out the long pauses, edit out every time you say 'um' or 'uh.' Cut out that one or two segments which just don't really work or tend to drag down an otherwise popping cadence. That is how long the video needs to be. Notice I'm being vague on numbers... because the length of the video isn't really based on numbers. It's based on the cadence and tone of the story you are telling.
Also... yeah 7-15 minutes is a good range (I'm long-winded lol, sorry).
The best way to do research; so first and foremost you need to continue making videos the way that you are. Don't change that. These videos are going to be for your core audience. Honor them by continuing to give them what they subscribed for. But add to your arsenal by also making researched videos.
Start by watching other YouTube videos. See what catches your attention. See what's trending on social media. Get an idea for the subject of your video. Then go to the Keyword Explorer and start researching the terms that people are typing in order to find the information they want on that subject. Experiment with a dozen different ways to search that exact same keyword phrase and try it with different modifiers. See which terms have the most traffic and least competition.
Then you have to decide what works best for you. You can play it safe, take the keyword phrases with the least amount of competition. Or if you have faith that you can beat out the competition maybe you aim for the keyword phrases with higher traffic volume.
Once this is complete script and film your video based on this research and throw it out there! Take some notes on what you did, what you experimented with etc. Come back after a week and see how your audience responded to your decisions. Come back again after a month, three months and a year to see what effects new audiences are having on your watch time and click-through rate. That is the real value in all of this; seeing what works and what doesn't. Take notes, learn from your successes and your mistakes and implemented changes on the videos going forward.
