Never had a native, coloured etc, but have been taught that for TBs, ISHs etc they should fall to about 2 inches below the hock. But definitely personsal preference- i personally like them quite short
I, as a guide, would put my arm under the dock and trim it so that it was around an inch under is hocks. That's what I was old anyway, as it mimics the way they carry their tails when being ridden. It depends on your taste though, some people like longer tails, I just find it easier to keep clean if it a bit shorter! xx
The length totally depends on the horse in question and what you want to do. Competition horses generally have shorter tails. M&M ponies if they are shown have the length that the breed standard dictates. Traditional coloureds are shown with full tails. Horses and ponies turned out for the summer are usually better off with a long enough tail to be an effective fly whisk. I suppose the best answer is that a tail should be as long or short as you want it to be.
A hand's width below the hock. A shorter tail is much easier to keep clean and looks smart.
I notice that racehorses and show horses (except M&M) have their tails kept short and I see a lot of eventers and dressage horses with longer tails than I would like. A long, thick tail must weigh a ton, especially when wet/muddy.