It really depends on the cover in your particular area,i.e; the height of surrounding branches & the lay of your land.
All things being equal 15'-20' is a good range; above 20' the shot angles do change quite a bit.
As an side:
I met a guy in the woods once during firearms season who had somehow managed to set a stand at what he said was 70' feet-it looked mile high to me; how he felt safe I have no idea but he did hunt it. A few years later that tree toppled during a serious ice storm. He had hoisted some sort of fiberglass seat w/ a 4 point harness up that pine and secured it as a lock on. Crazy.