etymonline says
window...Originally an unglazed hole in a roof, most Gmc. languages adopted a version of L. fenestra to describe the glass version, and Eng. used fenester as a parallel word till mid-16c. Figurative meaning "time period when something may be accomplished" first recorded 1967.
American Heritage Dictionary says
5. An interval of time during which an activity can or must take place: a brief window of opportunity for a space mission; a window of vulnerability during which the air force was subject to attack.
WORD HISTORY: The source of our word window is a vivid metaphor. Window comes to us from the Scandinavian invaders and settlers of England in the early Middle Ages. Although we have no record of the exact word they gave us, it was related to Old Norse vindauga, "window," a compound made up of vindr, "wind," and auga, "eye," reflecting the fact that at one time windows contained no glass. The metaphor "wind eye" is of a type beloved by Norse and Old English poets and is called a kenning; other examples include oar-steed for "ship" and whale-road for "sea." Recently we have restored to the 800-year-old word window a touch of its poetic heritage, using it figuratively in such phrases as launch window, weather window, and window of opportunity or vulnerability.
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gailr
