Largely this is a matter of taste, but most of the time people choose one of three options:
1. Just below the window. This will be cheaper as less fabric is used. It can also be more practical if the room is busy or long curtains could pose a hazard e.g. if a room has a fireplace near a window.
2. Just off the floor. Full length curtains can make a room seem taller and they show off a beautiful fabric. Just off the floor looks neat and provides a calming focal point.
3. In a puddle on the floor. As above this shows off a beautiful fabric. Puddled fabric looks less formal and really suits linens and other natural fabrics.
The length of curtain you need depends on where you have your pole. We rate this blog on how to hang curtains from Emily Henderson