Book IV Chapter 4 of Vishnu Purana describes Ikshvaku dynasty as:
"Rama and his brothers had each two sons. Kusha and Lava were the sons of Rama; those of Lakshman were Angada and Chandraketu; the sons of Bharata were Taksha and Pushkara; and Subáhu and Śúrasena were the sons of Śatrughna. The son of Kusha was Atithi; his son was Nishadha; his son was Nala; his son was Nabhas; his son was Puńdaríka; his son was Kshemadhanwan; his son was Deváníka; his son was Ahínagu; his son was Páripátra; his son was Dala; his son was Chhala; his son was Uktha; his son was Vajranábha; his son was Śankhanábha; his son was Abhyutthitáśwa; his son was Viśwasaha; his son was Hirańyanábha, who was a pupil of the mighty Yogí Jaimini, and communicated the knowledge of spiritual exercises to Yájnawalkya. The son of this saintly king was Pushya; his son was Dhruvasandhi; his son was Sudarśana; his son was Agnivarńa; his son was Śíghra; his son was Maru, who through the power of devotion (Yoga) is still living in the village called Kalápa, and in a future age will be the restorer of the Kshatriya race in the solar dynasty. Maru had a son named Prasuśruta; his son was Susandhi; his son was Amarsha; his son was Mahaswat; his son was Viśrutavat; and his son was Vrihadbala, who was killed in the great war by Abhimanyu, the son of Arjuna. These are the most distinguished princes in the family of Ikshvaku: whoever listens to the account of them will be purified from all his sins."
In short, the dynasty starting Lord Rama goes as:
Rama, Kusha, Atithi, Nisadha, Nala, Nabhas, Pundarika, Kshemadhanwan, Devanika, Ahinagu, Paripatra, Dala, Chhala, Uktha, Vajranabha, Sankhanabha, Abhyutthitaswa, Viswasaha, Hiranyanabha, Pushya, Dhruvasandhi, Sudarsana, Agnivarna, Sighra, Maru, Prasusruta, Susandhi, Amarsha, Mahaswat, Visrutavat, Vrihadbala.
Now Vrihadbala was killed by Abhimanyu in the Mahabharata war. This makes it clear that there are 31 kings starting Lord Rama to Vrihadbala, so, 31 kings (of Ikshvaku dynasty) ruled from Ramayana until Mahabharata.
Now, Rama is said to be of 24th Treta. Some references to support this are:
Treta yuge chaturvinshe ravane tapseh shakshyat |
Ram dashrtathi prapiye sagane shakyamiyeewan ||
[Vayu Puraan 70.88]
Chaturvinshe yuge chapi vishwamitra pure sare |
Loke ram iti khyate tejsah bhaskaropam ||
Chaturvinshe yuge vats tretayaam raghuvanshaje |
Ramo naam bhavishyami chaturvhayu sanatane ||
[Brahmand Puraan 18.104.22.168]
All of the above references are stating "Treta Yuge ChaturVimshe" i.e., Rama is of 24th Treta. Mahabharata is of 28th Dwapara and we are now in 28th Kali.
This means that the period from Ramayana to Mahabharata went as:
24th Treta + 24th Dwapara + 24th Kali +
25th Satya + 25th Treta + 25th Dwapara + 25th Kali +
26th (Satya, Treta, Dwapara, Kali) +
27th (Satya, Treta, Dwapara, Kali) +
28th Satya + 28th Treta + 28th Dwapara
Assuming 1 Chatur-yuga = 12000 years (4800 years of Satya, 3600 years of Treta, 2400 years of Dwapara and 1200 years of Kali), we get 54,000 years.
Now, here comes the debate whether 4800 years of Satya, 2400 years of Treta etc. are "Human-years" or "God-years":
Assuming they're God-years:
Then, 54,000 Deva-years = 54000
×360 = 19,440,000 i.e., around 1.9 crore years. This means Mahabharata happened 1.9 cr. years after Ramayana took place. As there are only 31 kings ruling, average life of one king comes to 1.9 crore
÷31 = around 6 lakh years!
i.e., those 31 kings lived an average life of 6 lakh years.
Assuming 54,000 as Human-years:
This answer actually suggests that they are Human-years. Then 31 kings would have an average age of about 1700 years. I also checked Linga Purana and in the chapter where ages are described it also calls 4800 years as Human-years.
So, whether it is human-years or deva-years, the average age of kings would be 1000+ years. Assuming it as Deva-years, their average age will be 6 lakh+ years!
BUT Srimad Bhagavatam suggests here that it is Deva-year:
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.
It uses the word Divya which suggests it referring to celestial years. In such cases as already discussed average life of those Kings would be around 6 lakh years.
If we go with Linga Purana (which suggests Human-years) and Mahabharata, average life would be about 1700 years if Ramayana took place in the 24th Treta.
But as discussed in the answer here, the Vedas prescribe 100 years (to 300 years) of life to Humans. In such a case, if Ramayana occurred in recent Treta (i.e., 28th Treta) then the average life of those kings would be around 193 years (if Rama was born in beginning of Treta) to 77 yrs (if Rama was born during end of Treta). This value is quite reasonable and is also in accordance with the Vedas.
Bhavishya Purana also suggests that Rama was born in 28th Treta.
An interesting thing to note in this huge figure is that if a king married or reproduced even after 20,000 years after his birth, he'll still be alive to see his 31st generation!
So, considering above calculations, I actually want to know:
What was the average age of those 31 Ikshvaku kings?
Did they live a life of about 6 lakh years (assuming Deva-years) or 1700 years (assuming 24th Treta Human-year) or 77 to 193 years (assuming recent i.e., 28th Treta)?
Does any scripture talk about the age of those kings?
Can we form a family tree of Ikshvaku kings from Ramayana to Mahabharata using data (ages of those kings) from various scriptures?
Do we at least know ages of few of those 31 kings?
Does any scripture record who among those 31 kings ruled in Kali-yuga of previous ages (i.e., 24th Kali, 25th Kali, 26th, 27th etc.)?