c. 49 CE
Life is long enough if you use it
Earliest we can find
This is the earliest source we can find for the idea. Said again in 5 books on our list: Michel de Montaigne, Essays (1580), Samuel Johnson, The Rambler (1751), John Lubbock, The Pleasures of Life (1887), Carl Hilty, Happiness (1891) and Oliver Burkeman, Four Thousand Weeks (2021).
“We do not have a very short time assigned to us, but we lose a great deal of it: life is long enough to carry out the most important projects”
Full text read.
Seneca, On the Shortness of Life 1.3, tr. Aubrey Stewart (1889).
Project Gutenberg eBook #64576, gutenberg.org/