When someone says brief incarceration what length of time is that approximately?
It depends on the context. Brief is a relative term, and the
fact that it is talking about incarceration does not change that. A
career criminal who has served several sentences may refer to a 5
year incarceration as brief, while a person who has never been to
jail before may refer to spending a week incarcerated as a long
sentence.
This is not a technical legal term that means anything outside
of regular usage.