The city of Taunton has a wide array of architecture ranging from the colonial period to modern times. There are numerous pre-Revolutionary War private homes within the city, the oldest of which is the Joseph Willis House on Worcester Street, dating to about 1688. The city has over one hundred buildings listed on the National Register of Historic Places. Perhaps the most impressive structure in the city is the towering Bristol County Superior Courthouse, built in 1894 and designed by Frank Irving Cooper. With its tall copper dome, the Superior Courthouse is visible from many surrounding areas. It was added to the National Register of Historic Places in 1978. Currently, the Courthouse Complex is undergoing a major expansion and renovation program. Other significant buildings in the city include some fine stone churches, including the First Parish Church , the Pilgrim Congregational Church  and St. Mary's Church  on Broadway. Downtown Taunton has a number of historic commercial blocks along Main Street, Taunton Green and Broadway, built during the period from about 1840 to 1920. Many large homes built by the wealthy industrialists and merchants of the late 19th and early 20th century line Route 44 both east  and west  of the city center, while a majority of the city is occupied by more modest wood-framed single and multi-family homes, many over 100 years old. Modern single-family subdivisions, mostly built since the 1950s, exist in the outskirts of the sprawling city. The Central Fire Station at 50 School Street is recognized as the oldest functioning station house in the United States. The historic Taunton City Hall is located adjacent to Church Green.
Answer this question: How many years did it take to build the historic commercial blocks?
80