Minecraft wiki entry for Clock, associated text: Replace X with a value that determines how long the item should last before despawning: 6000 - 20 × <seconds> (for example, 5940 for a 3-second despawn). Every game tick, this value increases by 1, and the item despawns when the value reaches 6,000. Normally, items start at 0 and last 5 minutes (6000 game ticks = 300 seconds = 5 minutes), but setting the item entity's initial Age changes that.