The textbook answer is 1 in for 10 ft if the wall is designed for active earth pressure. There's no reason that you are obligated to design for the active case, it's just an option. You could also design for the at-rest case wherein the wall would be designed for 0 in for 10 ft (if there is really a "0").
A wall that's 12 ft high would be tolerated for 1.2 in of horizontal movement at the top.
Just a few ideas. Not sure there's a real answer other than to design the wall to make your client happy - ha.
It also depends on whether this is a yielding (as previously alluded to) or non-yielding wall. If the diaphragm resists the load at the top of the wall, then the lateral deflection is much less than previously mentioned.
