For valances drop is usually expressed in inches. For curtains the drop may be a specific unit of measure or a category: to the sill, below the sill, to the floor, or puddled on the floor.

The drop you need will involve personal preference and placement of the rod. For a longer, vertical line, place the rod either just below the ceiling line and find a drop that goes all the way to the floor or longer to puddle the curtains. Although, most window treatments are traditionally hung a few inches above the frame. Also remember to consider the header height when considering rod placement.