So a ''day-time of Brahma's 12 hours'' equals 1000 Maha yugas, Catur-yuga or Divya-yuga which are -.
Satya Yuga is 1,728,000 human years,
Treta Yuga is 1,296,000 human years long,
Dwapara Yuga 864,000 human years
Kali Yuga 432,000 human years.
So a ''day and night 24 hour period'' of Brahma's time equals 8 billion 640 million human years.
Lord Caitanya and Krishna's appearance is very rare as they comes just ''once'' over that enormous period of time.
Srila Prabhupada – ‘’ Lord Kṛṣṇa and Lord Caitanya appear once in each day of Brahmā’’ (CC Adi 3.10 Text 10 Purport)
Krishna in His original Form comes at the end of every "one thousand" Dvarpara-yuga
Lord Caitanya ''only once'' in every "one thousand" Kali-yuga
The day-time of Lord Brahma is 4 billion 320 million human years long and, as already explained, there are no yugas Brahma's night time, which is also 4 billion 320 million years.
So a ''day and night'' 24 hour period of Brahma is 8 billion 640 billion years
CC Adi 3.10 Text 10
At the end of the Dvāpara-yuga of the twenty-eighth divya-yuga, Lord Kṛṣṇa appears on earth with the full paraphernalia of His eternal Vraja-dhāma.
PURPORT by Srila Prabhupada.
"Now is the term of Vaivasvata Manu, during which Lord Caitanya appears. First Lord Kṛṣṇa appears at the close of the Dvāpara-yuga of the twenty-eighth divya-yuga, and then Lord Caitanya appears in the Kali-yuga of the same divya-yuga.
Lord Kṛṣṇa and Lord Caitanya appear once in each day of Brahmā, or once in fourteen manv-antaras, each of seventy-one divya-yugas in duration.
From the beginning of Brahmā’s day of 4,320,000,000 years, six Manus appear and disappear before Lord Kṛṣṇa appears.
Thus 1,975,320,000 years of the day of Brahmā elapse before the appearance of Lord Kṛṣṇa. This is an astronomical calculation according to solar years".
Srila Prabhupāda - "Oh. Anyway, I give you the right figure, four million, according to American or English calculation, (laughter) 4,300,000 years and multiply it by one thousand. Then what it comes according to English calculation?
Paramahaṁsa - "4,300,000?".
Srila Prabhupāda - "That is twelve hours. And add again twelve hours, night. Then eight billion...? So Kṛṣṇa comes after this period. In one day, after one day of Brahma, He appears".
Devotee - "Śrīla Prabhupāda, does Lord Caitanya Mahāprabhu also appear every day of Brahma?"
Srila Prabhupāda - "Yes, following Kṛṣṇa. Kṛṣṇa comes in the Dvāpara-yuga. There are four periods of each yuga: Sattva, Tretā, Dvāpara, Kali. So Kṛṣṇa comes at the end of Dvāpara-yuga, and Caitanya Mahāprabhu comes in the Kali-yuga". (Lecture on SB 6.1.3, Melbourne, May 22, 1975)
How to understand all this -
We have to understand that one full day of Brahma from sunrise to sunset, has 1000 Divya-yuga (Maha-yugas) in it however, the twist is ALL those Maha-yugas occur ONLY the 12 hour day-time hours of Brahma.
Again, there are no Divya-yugas (Maha-yugas) during Brahma's 12 hour night-time.
This is important to understand.
The one thousand Divya-yuga or Maha-yugas happen during the "day-time" and take 4 billion 320 million human years to happen.
Now when the 12 hour night time starts, Brahma sleeps and does not create any Maha-yugas, also when Brahma sleeps during his night time hours, there is a partial annihilation lasting 4 billion 320 million human years, the duration of Brahma's night.
So when you add up the "day-time" of 4 billion 320 million human years and "night-time" of 4 billion 320 human million years, you get the full 24 hour day and night time of Brahma which is 8 billion 640 million human years.
The cycles of Divya-yugas (Maha yugas) starts again at the beginning of each "day-time and finish the same day-time at sunset"
In this way, Lord Krishna and Lord Caitanya Mahaprabhu only come "once" in a full day-night (24 hours) period of Brahma, or "once" every one thousand Maha-yugas.
Compiled by Gauragopala dasa ACBSP