Under the leadership of Donia , they employed guerrilla tactics and achieved several victories such as the successful siege of two Hollandic castles and the city of Medemblik. Donia also targeted ships that travelled the Zuiderzee and was very active in 1517, when he used his "signal ships" to attack ships in the region of the West Frisian coast, to which he also transported Geldrian forces, from the Duthcy of Geldern, setting them ashore at Medemblik. Donia bore a personal enmity to Medemblik and its inhabitants as, in earlier years, soldiers from Medemblik had cooperated with the Dutch army commanded by Duke Charles, the future Emperor. Donia sank 28 Dutch ships, earning him the title "Cross of the Dutchmen". The rebels also received financial support from Charles II, Duke of Guelders, who claimed the Duchy of Guelders in opposition to the House of Habsburg. Charles also employed mercenaries under the command of Maarten van Rossum in their support. However, when the tides turned against the rebels after 1520, Charles withdrew his support. Losing their financial support, the rebels could then no longer afford to pay their mercenary army. About the same time, the Arumer Zwarte Hoop also lost their leader. In 1519, Donia's health deteriorated. He retired to his farm where he died in 1520. He is buried in Sneek in the 15th-century Groote Kerk .
Question: How many years when Donia was very active in targeting ships in 1517 to when he died in 1520?
Answer: 3