One answer to this I got from Brahmarshi Satyadeva's 'Sadhana-Samara' (A Spiritual Commentary of the Devi-Mahatmya) which is now currently being published by the Gita Press.
There are three knots or Granthis in a human body and three types of body, one laid over another : The Brahma-granthi at the navel that corresponds basically to the Sthula Sharira or gross body where there is the Manipua Chakra, the Vishnu granthi that corresponds basically to the Sukshma Sharira or subtle body which is at the heart/throat (Visuddha Chakra) and the Rudra Granthi that corresponds to the Karana Sharira or the causal body which is within the eye brows or the Ajna Chakra.
The primal Avidya which is Anadi is the basis of the causal body.So Rudra/Shiva is called Anadi. He has NO birth. When the gross body dies, Brahma also dies. But the subtle body remains which takes birth again. So from the mind and the sanskaras that constitute the subtle body arises the gross body and Brahma is born from Vishnu. Only when at the pinnacle of the Spiritual Realisation, the subtle body is fully purified, the mind ceases to exist and so Vishnu merges with Rudra. This is Siddhi in the Saguna Brahman form. The aspirant is really liberated when His causal body is fully coeansed and destroyed.Then Rudra then merges with The Absolute. The aspirant experiences Nirvikalpa Samadhi.The rest is beyond description and ultimately Sahasrara is reached where resides Sadashiva, which gives the aspirant the state of Sahaja Samadhi.
Parama Shiva or Krishna is beyond these Who is the Parama Dhama.
Therefore according to this version lives of these three deities are not measured in human time but are entirely spiritual.
He explains in detail how this is implied by the Mantras of the Devi-Mahatmya that start withs Brahma worshipping Devi Kundalini and ends where the Rudra Granthi-bheda at last but one chapter of the Devi-Mahatmya.
Brahmarshi Satyadeva was a disciple of the lineage of Sri Trailanga Swami by the way.
Reference:Sadhana-Samaara, Brahmarshi Satydeva, Gita Press.