As low or high as you want it to go is the obvious answer.
However, I advise customers to not have the TV too high. Nothing worse than having to look up at a TV screen. IMHO, the highest the TV should be mounted is where the bottom of the screen is not much above eye level when you are sitting in your viewing position.If you are sitting a bit further away, you can afford to have the TV a little higher. The main thing is to not be looking up at the TV. Obviously you have to take into consideration how the TV looks in the room. If it's too low it will look "wrong". That seems to be a mistake a lot of people make, they choose the height based on how good it looks as far as the room is concerned, rather than viewing comfort. The classic case is where the customer wants the TV above a fireplace, because it looks good, but the seating position is fairly close and you're having to actually tilt your head backwards to watch TV. I advise against this.