It depends on whether your hydrangeas bloom on "new wood" or "old wood." These sites will help you out. Typically pruned in late winter to early spring depending on what zone you are in.
Early spring to the ground is what I've always seen. They will come back to last season's size.
Maybe it's different here. Both my grandmothers did this in the early spring and they grew back full size rather quickly. We are in zone 7a & 7b.
here is some more info: https://www.craftylittlegnome.com/when-should-hydrangeas-be-pruned/