In this task, you're given a statement, further information available on a particular linked term from the statement, and a question. Your job is to generate the answer to the question by using the information provided. If there is no clear answer obtainable, output 'none'.

Context: William the Conqueror, duke of Normandy, on the Norman invasion of England Link Information: William I (c. 1028 9 September 1087), usually known as William the Conqueror and sometimes William the Bastard, was the first Norman King of England, reigning from 1066 until his death in 1087. Question: How long had the duke of Normandy held that title when he commenced invading England?
Answer: 0