The books say 32 inches, but there is no reason for that to be a hard and fast rule. However if you make it a lot smaller you will have the impression of being a giant Alice in a tiny Wonderland. If you make it a lot higher, it will dominate the wall.
Wainscoting was originally created to protect the delicate lathe and plaster walls from things banging into them. The trim that tops off the wainscoting is called a "chair rail" and it actually had the function of catching the chair back right where it hits the wall. With modern drywall, protection is not much of a problem and wainscoting is just decoration today. However that chair rail can be useful to keep scratch marks off of both the wainscoting and the wall above. So the right answer to the question is: put the top of the wainscoting approximately 32 inches off the floor, but be sure that the chair rail actually catches your chair backs. With modern chairs you may want to raise, or more probably lower, the total height by one or two inches.