Intros/Outros

Outtro should be 10-20 seconds. Enough to use end cards and promote your work. Intros on the other hand... I don't know. If you do one it should 3-5 seconds long. But I don't see the point in them anymore. It loses audience retention if you use one. You should just state what the video is about at the beginning, introduce yourself and go straight into the content.
 
I'd say an intro is unnecessary, unless it's a bumper (1 or 2 second clip showing your brand name with little to no sound) or a intro to a specific series you might be doing. An intro should be under 15 seconds unless it's like a theme song or something. But keep in a mind a long intro might make people click away if they don't want to sit through it. An outro should be used to direct your viewers to subscribe and/or watch another video or two of yours. Outro's should be no longer than a minute imo.
 
An intro should be no more than 10 seconds and an outro should be 20 seconds because you can add a video pop up in the last 20 seconds of a video
 
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