Those who know Brahma’s day to last a thousand yugas, and his night (also) a thousand yugas — they know (the true measure of) day and night. (Bhagavad Gita 8.17)
We may think that when we go to Brahmaloka, the heaven of the Creator, then we are quite safe and secure from returning to this earth. But even Brahma goes to rest and then his world runs into chaos and we are thrown off. It is true, his life extends over many ages. We can hardly comprehend the period of his existence, but there is an end even to that. One day of Brahma is said to last four thousand million years and his life is one hundred of his years, but what of that? Even that will come to an end much sooner than we expect, because time is a relative idea. The greater the enjoyment, the more we lose sight of time. To an insect human life must seem inconceivably long. But to us, the human life seems none too long. Now, in Brahmaloka life will perhaps seem very moderate in duration.
So Vedanta always warns against the idea of going to heaven as not being the final end, but as so much time lost on the road to immortality. For we are sure to return to earth again, when our good karma is exhausted. Only the greatest jnanis do not come back even from Brahmaloka. They get mukti when Brahma gets free, after one hundred of his years. Then another Brahma is appointed. Some jnanis go to Brahmaloka because they are not yet perfect when they depart from here. In Brahmaloka they continue their practices in a fine, subtle body. These jnanis have no earthly desires. There is nothing to draw them back to earth. Therefore they get free from there. But those who go there through karma must return to earth. We must remember that Brahma, called the Creator, is not the maker of the plan of the world-scheme. He himself is included in that plan. He only carries out the plan. What is super-sensuous in the divine Mind, Brahma makes that perceptible. The world-scheme is eternal, but Brahma has an end when another soul takes his place and he becomes free. The plan of the world is revealed to Brahma during his meditation. First in him arises the desire to create. Then he sits in deep meditation and the world-plan is revealed to him. And according to that he creates. The future universe is already there. But it is involved, unmanifest. Then Brahma sets the wheel of samsara going and gradually everything evolves again.