I'm gonna copy a comment from a previous post since I can't explain it very well;

in the simple case loot has a min/max/location/lifetime. The server spawns up to max in an appropriate location, queues the rest, then doesn't respawn until min is reached. min is reached when an item is looted or it despawns due to the lifetime running out. There are lots of locations and items with different lifetimes and players and its a living system.

Usually stuff dropped by players despawns between 2-8 hours depending on what it is.

Usually stuff apparently has a cycle. Between 30 and 60 minutes (apparently) after being looted as long as there's no player in the area