For lets plays if you're doing 5 mins you will be suprised how quickly that goes. I tend to try and keep my videos at ten mins but because I act like an idiot and laugh and joke about things that have happened. It always seems to be close to 15mins. Which is a good cut off point. If I know the game is huge like fallout 4 then I do 30mins just to get through it. I know not many peeps will sit through that long but I have got a couple peeps dedicated enough to watch it all the way through.
If ever you do a lets play, say for 10mins for example. Record about 20 to 30mins worth. This will give you a lot of content to play with and a bigger chance of something funny happening or something random and awesome. Like you got a lucky shot in or there was a brief panic moment where other players were hunting you down but you managed to survive and take them out etc. Or in my case I managed to capture a funny moment where Dogmeat in fallout 4 randomly jumped off a building and killed/wounded himself. The pause between him jumping off and hitting the ground with a yelp was just too funny XD. Maybe I'm sadistic? lol
By doing that you can keep the good bits and cut the boring poop out, bringing it down to 10mins.
Its better to have too much conent then too little.