Never shorter than the hock ☺️
How short you want it to be
In the showing world if you take the end of the tail and wrap it round he leg you cut the point at which it reaches the hock
Have some put their hand under it's dock and cut 4 inches below the hock
Jumpers do a couple inches below to hock and hunters do a couple inches from the hooves but it can be different depending on what you prefer and what discipline you do. You never want short tails in the hunter but it may differ.
A could of inches below the hock wouldn't go any shorter x
A couple of inches up from hooves it's your own personal preference really x