Bhaktivinoda Ṭhākura said, anādi-karama-phale, pori' bhavārṇava-jale. We do not know when we have begun this conditioned life in this material world. You cannot trace. That is impossible. Because this life is not only in this creation, but it is coming from another creation. Suṣupti. Now the creation is going on since the birth of Lord Brahmā, and it will continue for so many millions of years. Again it will be annihilated.
As you will find in the Bhagavad-gītā, bhūtvā bhūtvā pralīyate (BG 8.19). This creation takes place exactly like your body, my body. The creation of this body takes place at a certain date. That is the beginning of history. But time is immemorial, I mean to say, eternal. It is all relative truth. The history is relative. Just like my life begins, this body begins somewhere in 1896—something like that—and it ends somewhere. That, that time limit is relative to my body. Similarly, there is Brahmā's body. That relative time is long, long duration.
As you'll find in the Bhagavad-gītā, Brahmā's life is described by Kṛṣṇa, sahasra-yuga-paryantam arhad yad brahmaṇo viduḥ (BG 8.17). This Satya, Tretā, Dvāpara, Kali, four yugas, it comes to about forty-three lakhs of years. So combine together, if you multiply it by one thousand, that is the duration of one day's life of Brahmā.