Everything in the world is relative. Our bodies, lives, intelligence, and everything else are all relative. To us an ant may seem to have a very short life, but for the ant his life is about a hundred years in duration. That hundred years is relative to the body. Similarly, Brahma, who lives fantastically long from our point of view, only lives a hundred years from his point of view. In the Bhagavad-gita (8.17) Krishna states: “By human calculation, a thousand ages taken together is the duration of Brahma’s one day. And such also is the duration of his night.” Thus Brahma, by these calculations, lives for so many millions and trillions of years.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), "The Science of Self-Realization", Page 261