King Pṛthu Was Born During the Cākṣuṣa Manu (the 6th Manu)

 According to the Śrīmad-Bhāgavatam (4.13), there are several generations before Prthu which proofs he lived during the sixth manvantara ie the reign of Caksusa Manu. 

1. Dhruva Mahārāja
2. Utkala (Dhruva's son) - ŚB 4.13.6
3. Vatsara - ŚB 4.13.11
4. Puṣpārṇa - ŚB 4.13.12
5. Vyaruṣa - ŚB 4.13.13 & ŚB 4.13.14
6. Sarvatejā - ŚB 4.13.14
7. Cākṣuṣa (he became the 6th Manu) - ŚB 4.13.15-16
8. Ulmuka -  ŚB 4.13.15-16
9. Aṅga (A very saintly king) - ŚB 4.13.17
10. Vena (The bad king) - ŚB 4.13.18
11. Pṛthu (Born from Vena's arms) - ŚB 4.13.19-20

There is a confusion that he was born in the 1st manvatara because he is related to Dhruva, and because Svayambhuva Manu (the 1st Manu of this day of Brahma, which is Brahma's 51st birthday) turned into a calf so Prithu could milk Bumi in the form of a cow. (ŚB 4.18.12)

Incidentally King Prithu is worshipping Lord Krsna, the Supreme Personality of Godhead.

ŚB 4.23.7
तितिक्षुर्यतवाग्दान्त ऊर्ध्वरेता जितानिल: ।
आरिराधयिषु: कृष्णमचरत्तप उत्तमम् ॥ ७ ॥

titikṣur yata-vāg dānta
ūrdhva-retā jitānilaḥ
ārirādhayiṣuḥ kṛṣṇam
acarat tapa uttamam

Synonyms

titikṣuḥ — tolerating; yata — controlling; vāk — words; dāntaḥ — controlling the senses; ūrdhva-retāḥ — without discharge of semen; jita-anilaḥ — controlling the life air; ārirādhayiṣuḥ — simply desiring; kṛṣṇam — Lord Kṛṣṇa; acarat — practice; tapaḥ — austerities; uttamam — the best.

Translation

Mahārāja Pṛthu underwent all these severe austerities in order to control his words and his senses, to refrain from discharging his semen and to control the life air within his body. All this he did for the satisfaction of Kṛṣṇa. He had no other purpose.

Purport by MGDAS

This proofs Krsna is the Supreme Personality of Godhead as this happened about 122 million years ago, as Prithu lived during the reign of Caksusa Manu, the sixth Manu. This is explained above. As we currently life in the 7th manvantara, during the reign of Vaivasvata Manu, as well as the 28th Maha-yuga of Vaivasvata Manu's reign. A maha-yuga is 4,320,000 human years. 

Sandhya (Transition Period): 1.728 million years (one Satya Yuga duration between Manvantaras).

27 Full Maha Yugas: (27 times 4.32 million years = 116.64 million years.

Current (28th) Maha Yuga progress: 3.89 million years (the sum of the elapsed Satya, Treta, and Dvapara Yugas, plus the ~5,125 years of the current Kali Yuga).

Total Estimated Time: ~122.26 million human years.