It depends on the canopy and the cover. I've hunted places where I climbed 25-30 feet before I was out of plain view. Other places, I've hunted 12-16 feet up and done fine.

The question shouldn't be "how high?" It should be "How hidden do I have to be?"

If you find a good tree that has 2-3 big trunks, you can position your stand inside the V, and be concealed very well. In that case, you won't need to be very high at all. You just have to pick the right time to draw your bow.

Another consideration is your scent. If you're confident that you're not putting off much human odor, hunting lower is fine. If you break a sweat on your way in, you might want to hunt a little higher to disperse your scent over their heads.