Lord Caitanya and Lord Krsna (as His original Form), both come just "once" in a day of Lord Brahma (once every 8 billion 640 million human years) to this Earth planet.

Lord Caitanya Mahaprabhu and Lord Krsna's appearance in this world is very, very rare.

Many take it for granted without realizing how rare Lord Caitanya and Lord Krsna's appearance actually is.

As they both come just "once" in a day and night of Brahmā (once every 8 billion 640 million human years) during Brahma's day light hours.

That is once out of every 1000 maha-yugas which all occur only in the 12 hour day-time period of Brahma's 24 hour day (sunrise to sunset)

The 24 hour day of Brahma is divided into day and night, only during the day do the one thousand maha-yugas exist.

At night while Brahma sleeps there are no maha-yugas but rather a partial annihilation of the material universe takes place.

Therefore 4,320,000,000 human years (4 billion 320 million years) equals the 12 hour daytime period of Brahma when all the 1000 maha-yugas appear.

Then another 4,320,000,000 (4 billion 320 million years) human years of Brahma's night occurs when a partial annihilation of this material universe takes place.

So a full 24 hour day of Brahma equals 8,640,000,000 human years (8 billion 640 million human years)

There are 4 yugas to a maha yuga-

1 - Satya-yuga
2 - Treta-yuga
3 - Dvapara-yuga
4 - Kali-yuga

Lord Krsna as His original Form appears at the end of just "one" Dvapara-yuga out of every 1000.

And Lord Caitanya appears near the beginning of "one" Kali-yuga with His Sankirtan Movement also out of every 1000 Kali-yugas just after Krsna's appearance.  

So both Lord Krsna and Lord Caitanya Mahaprabhu appear just "once" during Brahma's 24 hour day-night period.

So a day-time period from sun-rise to sun-set is made up of 1000 maha-yugas and is called a kalpa. 

And a night-time period from sun-set to sun-rise, where no maha-yugas exist, and a partial annihilation of the material universe occurs while Brahma sleeps, is also called a kalpa.

So there are two kalpas in a 24 hour day/night of Brahma which together equals 8 billion 640 million human years.

So Lord Caitanya and Lord Krsna's appearances are very, very rare.

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 divya-yuga (Maha-yuga), Lord Kṛṣṇa appears on earth with the full paraphernalia of His eternal Vraja-dhāma." (Caitanya Caritamrita Adi 3.10 Text 10)

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 (maha-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 (maha-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." (Caitanya Caritamrita Adi 3.10 Text 10)

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: 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)

A maha-yuga is also called a catur-yuga and divya-yuga which are- 

Satya-yuga,

Treta-yuga,
Dvapara-yuga,
Kali-yuga.

So as explained, there are no maha-yugas in Brahma's 12 hour "night-time" period (that also takes 4 billion 320 million human years) while Brahma sleeps and a partial annihilation takes place.