Minecraft wiki entry for Hopper_clocks, associated text: Each additional dropper stage can multiply the previous stage's clock period by up to 1,152 (twice the number of items a dropper can hold). Adding just one additional dropper stage increases the maximum clock period to over 10 years. In practice, this may only be needed for clock periods measured in weeks or months (longer than the 2-stage version can provide), generally on servers.