It depends on the tree canopy/terrain/obstructions. I've hunted White Oak flats that you needed to be close to 30' high to be out of view. Trees were like telephone poles with a 50' leaf canopy - wide open with no undergrowth.

There are other places where I can be completely hidden at 15' in a double- or triple-trunked tree. Every stand placement is different - and you have to make sure that you have an unobstructed shot, and can see into fields/hollows/benches/etc... Sometimes those intangiblesdictate your stand height as well.

On average, I'm about 21 feet from platform-to-ground. That's when my usual gear retriever runs out of rope. (24 ft, hooked to my waist).