According to Srimad Bhagavatam, the breathing in of Maha-Visnu though His mouth and the pours of His skin, happens when Brahma dies, taking 311 trillion 40 billion human years to complete.

The breathing in of Maha-Visnu through his mouth and the pores of His skin, is the process of annihilating the massive Brahmanda material universes. 

And when Maha-Visnu breaths out, He creates the next batch of Brahmanda material univeses also taking 31l trillion and 40 billion human years.

The annihilation of the Brahmanda material universes begins when Brahma dies, which is every 311 trillion and 40 billion human years, the life span of Brahma. 

Each Brahmanda material universe also lasts for the life time of Brahma which is 311 trillion 40 billion human years. 

Srila Prabhupāda - "Brahmā is born from the navel of Garbhodakaśāyī Viṣṇu, who is but a partial manifestation of the Mahā-Viṣṇu. 

Countless universes reside like seeds within the skin pores of the Mahā-Viṣṇu, and when He exhales, they all are manifest. 

In the material world we have no experience of such a thing, but we do experience a perverted reflection in the phenomenon of perspiration. 

We cannot imagine, however, the duration of one breath of the Mahā-Viṣṇu, for within one breath all of the universes are created and annihilated. 

Lord Brahmā only lives for the duration of one breath, and according to our time scale 4,320,000,000 human years constitute twelve hours of Brahmā's daytime, and the same time also is Brahma's 12 hour night time hours. Brahmā lives one hundred of his years which is 311 trillion and 40 billion human years. 

Yet the whole life of Brahmā is contained within one breath of the Mahā-Viṣṇu. 

Thus it is not possible for us to imagine the breathing power of the Supreme Lord. That Mahā-Viṣṇu is but a partial manifestation of Kṛṣṇa." (Teaching of Lord Caitanya [1975] Introduction) 

It makes sense because when Brahma dies his material universe and all the Brahmanda universes with Garbhodakashayi Visnu and Brahma deep inside them, are also destroyed as Maha-Visnu breaths in.

Mathmatically you can work it out based on information in Srimad Bhagavatam.

The gap in-between exhaling and inhaling is 311 trillion 40 billion human years.

The life span of each Brahmanda universe and its secondary material universe deep inside it, is one maha kalpa. i.e. 311 trillion 40 billion human years. As said above, this time span is also the duration of one breath of Maha-Visnu the creator of all material universes (Brahmandas) in the material creation that occupies 25% of the Spiritual Sky.

The other 75% is the eternal youthful ever fresh spiritual world's of the Vaikuntha planets and Krsna’s central Abode of Goloka-Vrindavana.

When Mahā-Viṣṇu exhales, an unlimited number of Brahmanda material universes emerge.

Maha-Visnu then He expands Himself as Garbhodakashayi Visnu, who builds His secondary material universe withvthe help of Brahma, inside of the greater Brahmanda material universe.

From Garbhodakashayi Visnu crestion, Brahma is born inside His secondary material universe. 

This secondary material universe is surrounded by 7 massive material layers of the Brahmanda that is 44 quadrillion 444 trillion 444 billions miles in diameter.

This is measured from the inside edge of the inner secondary material universe, to the surface of the Brahmanda which means the full diameter of the Brahmanda is twice the figure measured above. 

When Maha-Visnu inhales, all Brahmanda universes get sucked into His Body and are annihilated.

Brahma also comes to the end of his life and dies. This happens ever 100 years of Brahma, which is 311 trillion 40 billion human years.

So each time Maha-Visnu breaths fully out, it takes 311 trillion 40 billion years.

Then there is another 311 trillion 40 billion years when He breaths in.

In-between each breath of Maha-Visnu is when Brahma lives his 100 years (311 trillion 40 billion human years)

Then there is another 311 trillion 40 billion human years before Maha-Visnu exhales and creates the Brahmanda universes.

So, when Maha-Visnu exhales it takes 311 trillion 40 billion human years. Then it takes another 311 trillion 40 billion human years before Maha-Visnu begins to inhale.

The gap in-between exhaling and inhaling is also 311 trillion 40 billion human years.

So also between the breaths of Maha-Visnu, is also 311 trillion 40 billion human years.

There are two kalpas in a day/night of Brahma. 

Prakriti (primal matter) remains mixed for a maha-kalpa, life of Brahma that is 311 trillion 40 billion human years, and is followed by a maha-pralaya (great dissolution) of equal length that happens during Brahma's night-time. 

This happens as Maha-Visnu is breathing in which takes 311 trillion 40 billion human years.  

Therefore when Maha-Visnu breaths out the Brahmanda universes are gradually manifesting.

It does not mean the Brahmanda universes do not exists until Maha-Visnu fully breaths out.

And in-between Maha-Visnu's breathing is the life span of Brahma and his universe that is encased in the surrounding Brahmanda universe.

So, clearly the Brahmanda universes manifest as Maha-Visnu breaths out and not until He totally breaths out.

Maha-Visnu "breathing-in" is different because Brahma has already died and his universe is already annihilated.

So in this case, there is a period of time when there is no life active in the material universes which is 311 trillion 40 billion years.

Maha-Visnu breathing out does not mean the Brahmanda universes only appear when He has fully breathed out, no, the Brahmanda universes appear as He breaths out.×<>×.