In the days when chair rail was actually used as a bumper for chair backs, the answer would have been "as high as the chairs."
But chair rail now is used as more of a design element, than a practical device for protecting walls. You can look at chair rail height from either or both of two angles:
- Height from the Floor - Generally, in rooms with 8' high ceilings, you'll want to place chair railing at anywhere between 30" and 48". In most cases, though, your chair rail will be between 32" and 36" on-center from floor level.
- Relative Placement - But not all ceilings are 8' high. Some ceilings are 10' or 12'. In this case, the room could easily accommodate chair rail that is as high as 48".