Bhagavatapurana 3.11.20

3.11.20

sandhyā-sandhyāṃśayor antar yaḥ kālaḥ śata-saṅkhyayoḥ /
tam evāhur yugaṃ taj-jñā yatra dharmo vidhīyate //

19 & 20. The yugas respectively consist of four thousands, three thousands, two thousands, and one thousand years; the transition periods at the beginning and the end also respectively consist of eight hundreds, six hundreds, four hundreds and two hundreds of years; and the wise who know the fact call that a yuga which is between the transition periods counted by hundreds and only in relation to such yugas Dharma is laid down in the Holy books.