Stainless brake cables have an essentially indefinite life. The housings are
more likely to be a problem. Annual exams of the full length of the cable
for frayings, broken strands etc should be done. Non stainless brake cables
are more subject to aging problems: rust, breaking strands etc.
Der cables have a high stress area in the shifter where they wrap around
a small diameter, high tension drum that rotates when you shift so fraying
is especially likely within the shifter. This area should be inspected at least
annually or every few thousand miles for high mileage folks. Housings should
be changed as internal rusting can occur even with fully lined housings after
prolonged exposure, every other cable change is reasonable unless you are
a winter rider in northern climes or wet areas like Seattle.