Of the rebellion's leaders, the Earls of Northumberland and Westmorland had fled into Scotland. Northumberland was captured by James Douglas, 4th Earl of Morton, and turned over to Elizabeth in 1572, who had him beheaded at York. After having been hidden at Ferniehirst Castle, Westmorland escaped to Flanders, where he died impoverished. His family lost their ancestral homes and his wife, Jane Howard, also fled to the Continent. She lived the rest of her life under house arrest. Her brother, the Duke of Norfolk, was first imprisoned, then pardoned. He was imprisoned again following the Ridolfi plot in 1571 and finally executed in 1572. Norfolk's treason charges included "comforting and relieving of the English rebels that stirred the Rebellion in the North since they have fled out of the realm." Altogether, 600 supporters of Mary were executed, while many others fled into exile. Queen Elizabeth declared martial law, exacting terrible retribution on the ordinary folk of the Yorkshire Dales, despite the lack of any popular support for the Earls' Rising, with her demand for at least 700 executions. The victims of this purge were, as a contemporary account said "wholly of the meanest sort of people", so that hardly a village escaped the sight of a public hanging. In 1570, Pope Pius V had tried to aid the rebellion by excommunicating Elizabeth and declaring her deposed in the papal bull Regnans in Excelsis, but the document did not arrive until the rebellion had been suppressed. The bull gave Elizabeth more reason to view Catholics with suspicion. It inspired conspiracies to assassinate her, starting with the Ridolfi plot. In 1587, Elizabeth brought Mary, Queen of Scots, to trial for treason; she was convicted by the court and executed.
Question: How many years after Elizabeth had James Douglas beheaded at york did she have Mary, Queen of Scots executed?
Answer: 15