Some different issues here, it's an max limit on loose arrows in the world, the old ones are removed, it's also an limit on bodies, in the civil war attacks on Whiterun it goes over it and old bodies start to disappear. Now items you drop from inventory are not removed, they will stay forever unless somebody pick them up. Random containers are reset at respawn time and is the easiest way to test this, put some stuff who don't belong in it and see then it's goes away.