How long does the birth of a star take?
Stars are formed when large amounts of hydrogen pool up and the individual hydrogen atoms adhere to one another. Their individual mass must reach a critical point where their collective mass/gravity causes a nuclear reaction. At that point the star is born.
The size and speed at which a star forms is entirely dependent upon the amount of hydrogen available and its density relative to the forming star. The greater that number the faster the star forms and grows.