Jose wrote:
What is the protected area under an NDB approach course? Since the
course guidance is cruder than an ILS, I'd expect it would be wider, but
how much wider? I ask also in the context of when I should consider
myself "established" on course (for purposes of descent).

Jose

A whole lot wider than an ILS. From TERPs.

b. Area. Figure 57 illustrates the final approach primary and secondary
areas. The primary area is longitudinally centered on the FAC, and is 10
miles long. The primary area is 2.5 miles wide at the facility, and
expands uniformly to 6 miles wide at 10 miles from the facility. A
secondary area is on each side of the primary area. It is zero miles
wide at the facility, and expands uniformly to 1.34 miles on each side
of the primary area at 10 miles from the facility. When the 5 mile PT is
used, only the inner 5 miles of the final approach area need be considered.

The required obstacle clearance is also greater than other non-precision
approaches: 300 feet with FAF, 350 without.

The FAA does not define when established on an NDB or any other course.
It is implied by the IR PTS, but that is not a precise definition.

ICAO does define such things. But, those are not valid for FAA procedures.