Bathtub Faucet Handle Height Isn't Standardized
Bathtub faucet handle height is not specifically regulated by building code, unless the installation intends to meet the Americans With Disability Act of 1990. Handicap accessibility guidelines for buildings and facilities are stipulated under ADAAG requirements says the Access Board, author of much of the ADA language itself. Non ADA compliant installations are a matter regional preference.
Conventional Standards Are Often Used
According to American Standard US, bathtub spigots and handles are generally plumbed in 4 inches above the top of the bathtub rim. If a separate handle assembly controls a combined shower/tub unit as well, then the height of the plumb-in is raised 18 inches above the bathtub spigot plumb-in level by convention.
Raymond VinZant, of Bob Villa.com says that he sets the faucet handle height partially based on the actual size of the family, so that it may be comfortably reached by everyone. In addition, the only actual code requirement is the inclusion of a 2-inch air gap between the bathtub down spout and the top of the tub rim.