duration (n.)

"continuance in time; length of time during which anything continues," late 14c., duracioun, from Old French duration, from Medieval Latin durationem (nominative duratio), noun of action from past-participle stem of Latin durare "to harden," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." The old legalese phrase for the duration was popularized 1916 in reference to British enlistments in World War I.