Lord Caitanya Mahaprabhu comes to just one Kali-yuga out of every 1000 Kali-yugas.

This means Lord Caitanya Mahaprabhu comes once in a 24 hour day/night of Brahma, or "once" every 8 billion 640 million human years.

There are 1000 Maha-yugas, cycle of four yugas-

Satya-yuga,

Treta-yuga,

Dvarpara-yuga,

Kali-yuga.

In one-half day (day-time only) of Lord Brahma's day (sunrise to sunset), the 1000 Maha-yugas exist during that time.

There are no Maha-yugas during Brahma's night-time period from sunset to sunrise while Brahma sleeps.

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

Harinam Sankirtan is Lord Caitanya Mahaprabhu’s great gift to the particular Kali-yuga He comes too.

His appearance is very, very rare because as said above, Lord Caitanya does not come to every Kali-yuga during Brahma's day.

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

And then during Brahma's night when there are no Maha-yugas, another 4 billion 320 million years pass while Brahma sleeps and a partial annihilation goes on.

A Maha-yuga also called a Divya-yuga, is a cycle of 4 yugas-

Satya, 1,728,000 human years.

Treta, 1,296,000 human years.

Dvāpara, 864,000 human years.

Kali-yuga’s, 432,000 human years.

The length of each Maha-yuga is 4 million 320 thousand years.

It needs to be clearly understood that all Maha-yugas happen during Lord Brahma’s day-time period only, 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 He 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)

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 Dvāpara yugas)

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