Most scenes in a first draft begin about a page before they need to and end about half a page after they should. Greetings, arrivals, small talk and wrap-up are the writer clearing their throat, and readers charge you for every line of it.
Enter on the first line that could not be predicted. If a reader could guess what happens in the first eight lines, those lines are a summary of the scene rather than the scene. Cut to the moment the situation is already in motion.
Leave on the turn. The instant the scene's change has happened — the decision made, the lie told, the alliance broken — get out. The audience will happily assume the goodbyes. Ending late is worse than starting late, because it drains a moment you already earned.
Economy is also about count. Two scenes that do the same job become one better scene. Every scene should be answerable: what changed, and who now wants something different because of it?