Ephesians 5:16 says we should "[make] the most of the time [we have been given], because the days are evil." Other translations capture that phrase as "redeem the time." In other words, we are taught to take advantage of every opportunity to live for God.
Those opportunities are limited because our lifespans are limited. We only have so many trips around the sun allotted to us. That means we have a limited number of opportunities to share the Gospel with others