Longer than 10^34 Years
Modern physics explains that this whole universe will come to naught or will be completely dead in distant future such as trillion years multiplied by trillion times trillion or more than 10^33 years.
It is also notable that a proton, the major component of any atoms, is expected to collapse in 10^34 years. It means that there will be no elements and no non-dark materials or no ordinary substances, not to speak of molecules, chemicals, cells, and living things after 10^34 years.
So, in the excessive amount of time, the universe will be completely empty and void while it continues to expand to infinity. And at the end of time, the universe might finally disappear.
So, we live just in a very small time frame of the universe. Everything our ancestors and we have created and our descendants will create will come to nothing.
Indeed, Christ Jesus said: Lu:21:6: As for these things which ye behold, the days will come, in the which there shall not be left one stone upon another, that shall not be thrown down.
So, basically, our life-time is limited. Even this universe has its own finite life-span. Our pride and satisfaction acquired in life based on wealth and success will be meaningless. Even our love and hatred to others will be meaningless. But our faith and love in God will be meaningful if this universe disappears, since the God is above the material universe.
Yet our history is meaningful that the God sent Christ Jesus in it, specifically 2000 years ago.
The mankind might be extinct in the next 2000 years far before this universe collapses. But this history of the mankind is meaningful due to its connection with the God.
And after the collapse of this world and universe, the spiritual world should continue to exist, and thus Heaven continues to exist in addition to the hell. So life of our spirits will be longer than this universe.
Therefore, we should not think that happiness in this world is the most important thing, since it is truly nothing, compared with happiness in Heaven.
Life in Heaven or the hell will be longer than 10^34 years and more.
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Lu:1:19: And the angel answering said unto him, I am Gabriel, that stand in the presence of God; and am sent to speak unto thee, and to shew thee these glad tidings.
Lu:1:20: And, behold, thou shalt be dumb, and not able to speak, until the day that these things shall be performed, because thou believest not my words, which shall be fulfilled in their season.
Lu:1:21: And the people waited for Zacharias, and marvelled that he tarried so long in the temple.
Lu:1:22: And when he came out, he could not speak unto them: and they perceived that he had seen a vision in the temple: for he beckoned unto them, and remained speechless.