15" - 48" per ADA.gov
I go with 36" to center. Same as most door knobs/handles.
Department of Justice ADA Title III Regulation 28 CFR Part 36 (1991)
4.2.5* Forward Reach. If the clear floor space only allows forward approach to an object, the maximum high forward reach allowed shall be 48 in (1220 mm) (see Fig. 5 and Fig. 5(a)). The minimum low forward reach is 15 in (380 mm). If the high forward reach is over an obstruction, reach and clearances shall be as shown in Fig. 5(b).

4.2.6* Side Reach. If the clear floor space allows parallel approach by a person in a wheelchair, the maximum high side reach allowed shall be 54 in (1370 mm) and the low side reach shall be no less than 9 in (230 mm) above the floor (Fig 6, Fig. 6(a) and Fig (b)). If the side reach is over an obstruction, the reach and clearances shall be as shown in Fig 6(c).

(37) Side Reach
ABRA 6.19. ADAAG 205.1; 228.1-2; 308.3; 309.3.Compared to the current requirement, the revised side reach requirement will have a lower maximum (48" instead of the current 54") and higher minimum (15" instead of 9"). The requirement will apply to operable parts on accessible elements located on accessible routes, and to elements in accessible rooms and spaces. Elements and operable parts and controls that could be affected by the revised side reach requirements include: electrical outlets; thermostats; fire-alarm pull stations; card readers; keypads; coat hooks; window control hardware; paper towel dispensers and hand dryers in toilet rooms; ATMs; and at least one in each group of depositories, vending machines, change machines, and gas pumps (with a specified exception). (Elements may comply with either the side or forward reach requirement.)