Early in his career Richard Grenville had some minor posts in the royal household.  On the death of his father in 1523 he completed his father's term as Sheriff of Cornwall, a post he also held in 1526-7, 1544-5.  In between he was Sheriff of Devon in 1532-3.  He was Justice of the Peace for Cornwall from 1524 till his death and also in Devon, at Exeter from 1535 to 1547. Further afield he was Marshal of Calais, a post that required his residence there, from October 1535 to October 1540.  In 1544 Grenville accompanied the King to France as a commander in the English army. He was active in his Christian faith and openly favoured ‘God's word'. In the Prayer Book Rebellion of 1549 Grenville was called upon against the western rebels, and with a company of friends and followers he defended Trematon Castle. When they were unsuccessful in defending the castle, he and his wife were held in custody in Launceston prison. Grenville contracted the illness probably while in gaol and died from it on 18 March 1550.  He was buried at Kilkhampton five days later.
Question: How many years was Richard Grenville a Justice of the Peace for Exeter for?
Answer: 12