In the US, 48" seems to be standard. Whether your say 48 to the bottom of the box, top of the box, or middle of the box is a matter of personal preference. If it's commercial work, there are ADA requirements to contend with. The ADA guidelines are free online.
http://www.access-board.gov/adaag/html/adaag.htm
Related to electrical, This if from NECA-1
11.1 Recommended Outlet Mounting
Heights
Table 3 describes typical mounting heights for various
outlets and control devices. All heights are measured
from finished floor to centerline of device.
These heights comply with the requirements of the
ADAG (American with Disability Act Guidelines).
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