According to the Manusmriti, one of the earliest known texts describing the yugas, the length is 4800 years + 3600 years + 2400 years + 1200 years, for a total of 12,000 years for one arc, or 24,000 years to complete the cycle, which is one precession of the equinox).
According to Bhagavata Purana 3.11.19,