30-36" but it should be based on your ceiling height. Approximately 1/3 of the ceiling height. Of course if you have tall backed chairs then the chair rail should be at the height to protect the wall from the chair backs.
"Determine the height of your chair rail. You can place a chalk line or strip of masking tape on the wall to help visualize the height while you decide. It will probably be somewhere between 30 and 36 inches (75-90cm) high, though some go higher for dramatic effect. Here are other factors to consider.
The height of your chairs. Whether or not you are installing the chair rail to prevent damage to the wall, compare its height to your furnishings. It will help achieve a unified look and feel.
Windows in the room. Will you break the chair rail for windows, or go below them?
The height of any paneling that will go above or below the chair rail. If the chair rail will mark the top edge of wainscoting, for instance, simply follow the top edge of that material." - http://www.wikihow.com/Install-a-Chair-Rail