Śrīmad-Bhāgavatam · Text 3.11.18
Who is speaking and story frame
मैत्रेय उवाच
कृतं त्रेता द्वापरं च कलिश्चेति चतुर्युगम् ।
दिव्यैर्द्वादशभिर्वर्षै: सावधानं निरूपितम् ॥ १८ ॥
maitreya uvāca
kṛtaṁ tretā dvāparaṁ ca
kaliś ceti catur-yugam
divyair dvādaśabhir varṣaiḥ
sāvadhānaṁ nirūpitam
Maitreya said: O Vidura, the four millenniums are called the Satya-, Tretā-, Dvāpara- and Kali-yuga. The aggregate number of years of all of these combined is equal to twelve thousand years of the demigods.
Read Śrīla Prabhupāda's purport on Vedabase.io