Video length

There is no correct figure. The useful question is whether every minute is doing work, and a 25 minute video with nothing padded holds people better than a 9 minute one carrying six minutes of throat-clearing. Length also sets expectations before anyone presses play, because the duration in the corner of the thumbnail is part of the packaging, and a viewer scrolling on a phone weighs a 40 minute commitment differently from a 12 minute one. Write the idea first, then cut anything you would skip yourself. The runtime that survives is the right one.