Fyi:fromhttp://www.ehow.com/how_5192598_build-window-casing.html.
A window casing is the trim molding that encases a window. It originates from the windowsill and wraps up and around the window. The stool and apron (windowsill) are generally considered separate from the actual casing. Some windows in modern homes only have windowsill moldings, and the area surrounding the windows are faced in sheet rock and plaster. To case this type of window, the windowsill often needs to be replaced so that the stool ends can be extended along the wall on either side of the window to receive the new casing. The window casing should sit on top of the stool, and the stool should extend one half inch beyond the edge of the new casing.
with that said maybe mid xx but thats just me.