Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sexennĭum, ii, n. [sexennis], a period of six years, six years: puer subripitur Sexennio prius quam moritur pater, Plaut. Poen. prol. 67: tribuni plebis tulerunt de provinciis contra acta Caesaris, ille biennium, hi sexennium, Cic. Phil. 5, 3, 7; so id. Div. 1, 44, 100; id. Att. 6, 1, 5.