Seneca's quote "Longa est vita, si plena est" has been translated in many ways. One popular translation is:
"Life is long if you know how to use it."
Here's a bit of context:
In philosophy, Seneca expounds ethical doctrines of Stoicism, which was dominant at his time. As a proponent of Stoicism, Seneca taught the importance of living a life free from desire and negative emotion, and in accordance with nature. He often used life itself as the most precious resource and the sole basis of all other values.