After British imperial forces withdrew from most Canadian locations in 1870-71, the militia authorized the creation of two batteries of garrison artillery which provided garrison duties and schools of gunnery. "A " Battery School of Gunnery was established at Tête-de-Pont Barracks and other locations in Kingston . These batteries were known as the Regiment of Canadian Artillery.  When this regiment evolved into the Royal Canadian Horse Artillery , its headquarters was at the Tête-de-Pont Barracks from 1905 to 1939. After the RCHA left for operational duties during the Second World War, the fort was used as a personnel depot. In 1939 the site of the fort again became known as Fort Frontenac. Canadian Army staff training began at Fort Frontenac when the Canadian Army Staff College moved to the fort from the Royal Military College in 1948. The college is now known as the Canadian Army Command and Staff College. Fort Frontenac was also the location of the National Defence College until 1994. Fort Frontenac was designated a National Historic Site of Canada in 1923.
Question: How many years did it take for the British imperial forces to withdraw from most of Canada?
Answer: 1