When it comes to discussing the length of a person's life, many different words could be used as synonyms for "life's duration." Some possible options might include phrases like "lifespan," "life expectancy," or "time on earth." Other descriptive terms could include "years lived," "existence length," or "lifetime length." Each of these expressions conveys the idea of living a certain number of years and experiencing all that life has to offer within that time. Ultimately, the specific term used to describe "life's duration" may depend on the context of the discussion and the tone of the conversation.