Lord Caitanya Mahaprabhu comes to just "one" Kali-yuga out of every 1000, which is once in a day/night of Brahma, which is once every 8 billion 640 million human years.

There are 1000 Maha-yugas (cycle of four yugas) in one-half day (day-time only) of Lord Brahma's day (sunrise to sunset) and no Maha-yugas during Brahma's night.

In this present age of Kali-yuga, Harinam Sankirtan (the chanting of the Hare Kṛṣṇa mantra) is the greatest gift given to this degraded period of earth history.

Harinam Sankirtan is Lord Caitanya Mahaprabhu’s great gift to this particular yuga, His appearance is very, very rare because Lord Caitanya does not come to every Kali-yuga during Brahma's day.

Lord Caitanya Mahaprabhu only comes to just "one" Kali-yuga out of every 1000, which is once in a day/night of Brahma. or once every 8 billion 640 million human years.

There are 1000 Kali-yuga’s in one-half day (day-time only) of Lord Brahmas time (sunrise to sunset)

It takes 4 billion 320 million human years for 1000 Maha-yugas to occur during Brahma's day from sunrise to sunset.

There are no Maha-yugas (also known as a Divya-yugas) during Brahma's night (sunset to sunrise).

A Maha-yuga or Divya-yuga is a cycle of 4 yugas-

Satya, 1,728,000 human years

Treta, 1296,000 human years 

Dvāpara, 864,000 human years 

Kali-yuga’s, 432,000 human years.

Making the length of each Maha-yuga 4 million 320 thousand years.

1000 Maha-yugas are in a day of Brahma (during Brahma's day-time hours), and no Maha-yugas exist during Brahma's night.

In other words, all Maha-yugas happened during Lord Brahma’s day-time period, taking 4 billion 320 million human years to occur.

Then, add the same duration of time (4 billion 320 million human years) for Brahma’s night (sunset to sunrise) to make a full day of 8 billion 640 million human years.

So we understand that Lord Caitanya's appearance in just one Kali-yuga, (the same Kali-yuga we are in now), is very, very rare as comes "once" to this earth every 8 billion 640 million human years.

This means Lord Caitanya does not appear in the other 999 Kali-yugas during Brahma’s day (sunrise to sunset)

During Brahma's night, while he sleeps (sunset to sunrise), the material universe experiences a partial annihilation. This happens over a period of 4 billion 320 million human years (Brahma's night)

So one "day-night 24-hour period" of Brahma equals 8 billion 640 million human years as already explained above. Lord Caitanya Mahaprabhu comes “once” in that 24-hour day of Lord Brahma at the beginning of a Kali-yuga.

Krsna, as His original Form in Vrindavana, also comes to this earth planet once every 8 billion 640 million human years, at the end of Dvāpara-yuga (coming to just one Dvāpara-yuga out of every thousand)

So both Krsna and Caitanya appear only "once" in a day of Brahma (every 8 billion 640 million human years)

Krsna, in His original Vrindavana  Form, also comes "once" in a 24-hour day of Brahma at the end of a Dvāpara-yuga.***.