Lord Krsna as His original form and Lord Caitanya Mahaprabhu only come to ONE Maha-yuga out of every thousand during Brahma's day.

During the 28th Maha-yuga of the Vaivasvata Manu (7th of 14 Manus), 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 Mahaprabhu appear ONLY "once" in a 24 hour day of Lord Brahmā but only during the day-time hours.

Out of the one thousand Maha-yugas in each day of Brahma, both Lord Krsna and Lord Caitanya Mahaprabhu only come to one Maha-yuga.

Krsna as His original Vrindavana form appears at the end of a Dvāpara-yuga and Caitanya appears at the beginning of the following Kali-yuga.

Lord Krsna and Lord Caitanya do not appear in all the other Dvāpara-yugas or Kali-yugas during Brahma's day.

Both Lord Krsna and Lord Caitanya Mahaprabhu appear on this Earth Globe during the 28th Maha-yuga of the present 7th Manu called Vaivasvata.

There are 71 Maha-yugas in a Manu, and there are 14 Manus in each day of Lord Brahma (from Sun-rise to Sun-set) 

There are no Manus or Maha-yugas during Brahma's night while he sleeps when a partial annihilation takes.

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

In our current kalpa (day-time hours of Brahma), these fourteen Manu's reign in succession-

1 - Swayambhu Manu

2 - Swarochisha Manu

3 - Uttama Manu

4 - Tapasa/Tamasa Manu

5 - Raivata Manu

6 - Chakshusha Manu

7 - Vaivasvata Manu (The current Manu we are in (the 28th Maha-yuga) This means it is just before midday during Brahma's day)

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.

The life of Brahma is 311 trillion and 40 billion human years long, which is 100 of his years.

The current age of Brahma is 50 years and one day old in his years, which is around 155 trillion, 521 billion, 972, million, 944 thousand human years old.

And it is near midday in Brahma's day.

There are 4 ages (yugas) in this earth globe's Bhurloka planetary system which keeps circulating-

Satya Yuga - 1 million 728 thousand human years long.

Treta Yuga - 1 million 296 thousand human years long.

Dwapara Yuga - 864 thousand human years long.

Kali Yuga - 432 thousand human years long

The four yugas together are called a Maha-yuga and are 4 million 320 thousand human years long.

Presently we are just over 5 thousand years into Kali-yuga which means there are 427 thousand human years left in Kali-yuga.

It is also explained by Srila Prabhupada that because of Lord Caitanya Mahaprabhu's appearance in this Kali-yuga, there will be a 10 thousand year old Golden age of His Sankirtan Movement in Kali-yuga.  

However, Lord Caitanya only comes to ONE Kali-yuga out of every thousand, or once in a day of Brahma, His appearance is very rare.

Also Lord Krishna as His original Vrindavana form appears only once out of every thousand Dvāpara-yugas or once a day of Brahma.

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)

As said above, these 4 yuga cycles are called a maha (in english means great/big) or divya (divine) yuga. 

One thousand such cycles forms one day-time period of Brahma which is 4 billion 320 million years long

And for night, the same period of 4 billion 320 million human years also occurs but there are no Maha-yugas while Brahma sleeps and a partial annihilation occurs.

So one day and one night of Brahma are each 4 billion, 320 million human years long.

And one day/night together of Brahma equals 8 billion 640 million human years long.

Also each day/night of Brahma equals two "kalpa" that each constitutes 4 billion 320 million human years long. 

During Brahma's day-time, all life forms exists in the universe, at night-time no forms of life exists. 

During Brahma's night all individual jiva souls merge dormantly into Maha Vishnu until the next day begins. 

So one complete "day and night" of Brahma equals 8 billion 640 million human years long.

Brahma lives for 100 of his years. Each year of Brahma has 360 days and same number of nights. 

The total life span of Brahma is 311 trillion 40 billion human years. 

This period is also called "maha kalpa" and the life span of this single universe that exists deep inside our outer surrounding massive Brahmanda universe.

Therefore one "maha kalpa" (the life of Brahma) is 311 trillion 40 billion human years long. 

This time span is also the duration of one breath of Maha Vishnu who creates ALL of billions of Brahmanda universes that each have a secondary smaller material universe deep inside them where Maha Vishnu expands as Garbhodakashayi Vishnu. 

From Garbhodakashayi Vishnu Lord Brahma appears and builds (creates) the many planetary systems in their universe inside their Brahmanda.

Each "secondary material universe" deep inside their Brahmanda, is of different sizes and have a different number of planetary systems.

The size of each "secondary material universe" inside their Brahmanda is based on the number of heads Brahma have, some have 4 heads like our Brahma, others have 10, 20, 50, 500, 1000, 10,000, 1000,000 and even 10,000,000!

When Maha Vishnu exhales, thousands of Brahmanda universes emerges and one "Brahma" is born inside a secondary universe that exists in each Brahmanda universe. 

When "Maha Vishnu" inhales all the Brahmanda universes merge back into His Body and Brahma simultaneously dies.

The same Brahmanda universes again emerge when Maha Vishnu breath outs which happens 311 trillion 40 billion human years after He breathed them in.

This means all the jiva souls not qualified to return back home back to Godhead, remain dormant in Maha Vishnu's Body for 311 trillion 40 billion human years, and then carry on in the new manifestation from where they left off in the previous Brahmanda creation.

This cycle of Brahmanda universes manifesting from the breathing and pores from his body (creation and annihilation) in the material creation from Maha Vishnu is eternal. 

How old is our single universe -

It is presently 155 trillion, 521 billion, 972, million, 944 thousand human years old (Brahma is 50 years and one day old in his years).

In one day of Brahma there are 14 "Manvantars" (Manus), each "Manvantar" is divided into 71 "maha-yuga" cycles. 

So there are 14 × 71 = 994 Maha yuga cycles.

Remaining cycles (one day is Sun-rise to Sun-set) of Brahma contains 1000 maha-yuga cycles) are used to fill gaps between Manvantara. 

Before and after each manvantara (called as "sandhya" and "sandhyamsa" respectively), there is a gap of 1 million 728 thousand human years (age of Satya Yuga)

Total number of gaps are 15 (since there are 14 manvantaras). 

15 × one million 728 thousand brings the figure up to 1000 Maha-yugas during Brahma's day-time hours.

Therefore it works out there is a total of 1000 maha-yuga cycles in one brahma's "day-time" hours.

According to the Vedic texts, current age of Brahma is 50 Brahma years and 1 brahma day (we are in the 1st day of 2nd half of brahma) and we are in the seventh "manvantara", in the 28th turnover of its 71 yuga cycles. 

In this cycle, we are in the start Kali Yuga that lasts for 432,000 human years, we are 5000 years into Kali-yuga.