How deep should you insert a golf tee?

Honestly it is personal preference. Although as a general rule when using your driver half of the ball should be above the top of the driver, and the same for your woods. You should tee your long irons up so the ball is roughly in the middle of the club face and for shorter irons and wedges you should tee it up so that the ball is just off the ground so you can stike down on the ball, compress it and get some spin.