This is dependent on your sail clew height.
If you are cruising then mount the spinnaker pole ring at approximately the same height as the clew of your spinnaker when flying it with the pole horizontal to the water.
For symmetrical spinnakers, this can be around 4 to 5 feet (1.2–1.5 m) above the deck, but it varies by boat.
When racing, boat owners will use a track so that the pole can be adjusted up or down on the mast as sail trim requires.