Why did slavery start in the United States? How long did it last for?

Up until 1676, indentured servants were the primary source for labor in the New World.  Slavery, as noted, had begun almost as soon as the English colony of Virginia began to be settled, but slaves were a poor source for labor -- they were very expensive and they usually died within 7 years of arriving. Indentured servants were much cheaper; they too usually died within 7 years of arrival, but their sponsor was out much less money, as it was cheaper to pay for an indentured servant's passage across the Atlantic than it was to purchase a slave for life.

Tobacco became the primary export; its cultivation was labor intensive.

Slavery began in earnest after Bacon's Rebellion, in 1676.  The indentured and newly freed servants and small poor white farmers stormed various plantations and destroyed what they could in protest against the abusive practices of the large scale farmers.  The rebellion was put down, but afterwards racism was legally enshrined in a series of acts that were designed to give the poor white farmers more of a stake in the agrarian economy by excluding black farmers (up until this time, there were free black farmers who owned slaves!) Conditions had improved so that life expectancy was greater; slaves, rather than troublesome indentured servants, became the mainstay labor force for nearly the next 200 years; racism upheld the social and political structures in the South and has finally rapidly diminished in our own time.

American Slavery, American Freedom, 1975 HW Brands.