Ars longa, vita brevis
The translation into Latin of part of a quotation by the Greek 'Father of Medicine' - Hippocrates.
This is one of those rare phrases in which the meaning is more debated than the origin. What is usually understood by 'Ars longa, vita brevis' is something along the lines of 'art lasts forever, but artists die and are forgotten'.
That is questioned by some, who say that it is a misinterpretation based on a misunderstanding of the translation of 'ars' as 'art'. If we accept that the Latin term 'ars' is equivalent to the Greek 'techne' and that, consequently, 'ars' is better translated into English as 'skill' or 'craft', we may opt to interpret the phrase differently. The full quotation, in Latin, is
"Ars longa, vita brevis, occasio praeceps, experimentum periculosum, iudicium difficile."
This can be rendered into English as 'life is short, the art (craft/skill) long, opportunity fleeting, experiment treacherous, judgement difficult'.
That would lead us to interpret the meaning as 'it takes a long time to acquire and perfect one's expertise (in, say, medicine) and one has but a short time in which to do it'.