Minecraft wiki entry for Hopper_clocks, associated text: This Hopper timer is basically just a self-locking EHC with trigger input and a FED to detect the end of the full timer period.
So it needs to be started by activating the input (= interrupting the self-locking circuit) and will stop on its own after one full clock period. Unless of course the input is powered continuously which will turn it into a regular EHC but with trigger outputs.