There's no single right length, but there is a reliable way to decide: let the moment dictate the cut, then check it holds attention from the first second.

A few rules of thumb

Open on the point. If the first few seconds need context, the clip probably starts too early.
Cut at a natural end. End on a complete thought rather than trailing off.
When in doubt, shorter. It's easier to watch a tight clip twice than a long one once.

Let Captos do the first pass

Captos returns clips at sensible lengths so you're choosing between options rather than building each cut from scratch. Trim further whenever a clip feels slow.