How long living-room curtains should hang in relation to the bottom of the window is a matter of personal preference. Living-room curtains are readily available in a range of standard lengths, allowing you to choose curtains that meet your wants and needs.
A common length for living-room curtains is floor-length. Curtains that hang past the window frame to the floor give the living room a finished look.
If your decorating style leans toward casual, you might prefer window-length curtains, which should extend to just below the window frame. This length also works well if there are radiators, baseboard heaters or furniture pieces below the window.
For formal living rooms, curtain puddling provides an elegant look. Create puddling by purchasing living-room curtains with an extra 4 to 6 inches of fabric beyond floor-length and allowing it to fall into gentle folds, or "puddles," on the floor.