What does a hopper clock do?
An n-hopper-loop clock consists of a loop of hoppers moving a single item around which occasionally powers a comparator output. This clock runs while the input is off, and turns its clock signal output off when the input turns on. The clock period will be N × 0.4 seconds, where N is the number of hoppers.
How long does it take for a hopper to empty?
After collecting an item (or stack of items), a hopper waits 4 redstone ticks (0.4 seconds, barring lag) before attempting to collect again.