The National Labor Union , founded in 1866, was the first national labor federation in the United States. It was dissolved in 1872. The regional Order of the Knights of St. Crispin was founded in the northeast in 1867 and claimed 50,000 members by 1870, by far the largest union in the country. A closely associated union of women, the Daughters of St. Crispin, formed in 1870. In 1879 the Knights formally admitted women, who by 1886 comprised 10% of the union's membership, but it was poorly organized and soon declined. They fought encroachments of machinery and unskilled labor on autonomy of skilled shoe workers. One provision in the Crispin constitution explicitly sought to limit the entry of "green hands" into the trade, but this failed because the new machines could be operated by semi-skilled workers and produce more shoes than hand sewing.
Question: How many years did the National Labor Union operate for until it was dissolved?
Answer: 6