FA10:
Formally the coverage area is 25nm for the localizer and 10nm for the glideslope.
In the U.S. it is 18 nm within 10 degrees, and 10 miles over 10 degrees to 35 degrees.
If a greater distance is needed, then frequency protection has to be assessed, and a flight inspection performed for the required greater distance (known as extended service volumne of "ESV") The glideslope is flight inspected to the P-FAF.
The localizer parameters are covered by the FAA in the U.S. Aeronautical Information Manual, Section 1-1-9 b. 5.