Lord Krsna and Lord Caitanya come only "ONCE" to this universe every 8 billion 640 Million human years.

Lord Caitanya Mahaprabhu is the Golden Avatara for this age of Kali-yuga.

However, Lord Caitanya does NOT appear in every Kali-yuga, He only appears in ONE Kali-yuga out of every thousand.

Just like Lord Krsna in His original form, also only appears in ONE Dvāpara-yuga out of every thousand.

Lord Krsna appears in the Dvāpara-yuga just before the Kali-yuga Lord Caitanya appears in.

This means both Lord Krsna and Lord Caitanya come ONLY "once" every 8 billion 640 Million human years, which is a "day/night" 24 hour period of Lord Brahma time.

Srila Prabhupada – "Lord Kṛṣṇa and Lord Caitanya appear "once" in each day of Brahmā’’ (Caitanya  Caritamrita Adi 3.10 Text 10 Purport)

Srila Prabhupada –"At the end of the Dvāpara-yuga of the twenty-eighth Maha-yuga (during the 7th Manu known as Vaivasvata Manu), Lord Kṛṣṇa appears on earth with the full paraphernalia of His eternal Vraja-dhāma". (Caitanya Caritamrita Adi 3.10 Text 10)

Therefore during the 28th Maha-yuga of the Vaivasvata Manu (the 7th Manu of 14), Lord Kṛṣṇa first appears at the close of the Dvāpara-yuga and then Lord Caitanya appears in the Kali-yuga of the same Maha yuga.

Lord Kṛṣṇa and Lord Caitanya appear ONLY "once" in a day of Brahmā during the 7th Manu out of the fourteen, called Vaivasvata Manu.

First Lord Kṛṣṇa appears at the close of the Dvāpara-yuga of the twenty-eighth Maha-yuga in the Vaivasvata Manu (the 7th Manu), and then Lord Caitanya appears in the Kali-yuga of the same Maha yuga in Vaivasvata Manu.

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: Satva, Tretā, Dvāpara, Kali. So Kṛṣṇa comes at the end of Dvāpara-yuga, and Caitanya Mahāprabhu comes in the Kali-yuga". (Lecture on Srimad Bhagavatam Canto 6 Chapter 1 Text 3 - Melbourne, May 22, 1975)

Our Kali-yuga is very rare because of Lord Caitanya Mahaprabhu's appearance.

Also Lord Krsna, as His original form, appears only in the Dvāpara-yuga just before the Kali-yuga Lord Caitanya Mahaprabhu appears in.

Therefore Lord Krsna also does not appear as His original Vrindavana form in every Dvāpara-yuga, He only comes to ONE Dvāpara-yuga out of every thousand as said above.

Both Lord Krsna and Lord Caitanya Mahaprabhu only come "ONCE" in every "day/night" of Lord Brahma (between Sun-rise and sun-set) where all the 1000 Maha-yugas are active.

There are no Maha-yugas during Brahma's night (from Sun-set to sun-rise).

In our current kalpa (day-time hours of Brahma) there are fourteen Manus-

1 - Swayambhu Manu
2 - Swarochisha Manu
3 - Uttama Manu
4 - Tapasa/Tamasa Manu
5 - Raivata Manu
6 - Chakshusha Manu
7 - Vaivasvata Manu (the current one)
8 - Savarni Manu
9 - Daksa Savarni Manu
10 - Brahma Savarni Manu
11 - Dharma Savarni Manu
12 - Rudra Savarni Manu
13 -Raucya or Deva Savarni Manu
14 - Indra Savarni Manu

Lord Brahma's 100 year life span in human years is 311 trillion and 40 billion years.^^.