Minecraft wiki entry for Hopper_clocks, associated text: Specs:
9x5x2 (90 block volume)
flat
clock period: 8 ticks to 256 seconds (4m16s) [note 1]
the right hopper contains the counter items (of the stackable-to-64 type).
four different pulse outputs possible:
F once after the whole timer period.
H at the half way point.
P/p every time the related comparator measures a level change in its hopper.