The more you
use, the less likely it will be considered fair use.
If you use the "heart" or "essence" of a work,
it is less likely your use will be considered fair.
Does the work
appeal to the same audience as the original? If the answer is yes,
it will likely be considered an infringement
If you use something on one occasion within one semester or part of a semester it is more likely to be seen as fair use. If you use something repeatedly, it's less likely to be considered fair use.
In General, What Counts as Fair Use?
use for a limited period of time
Copying the
same works from semester to semester.
Copying the
same material for several different courses at the same or different
institutions.
Copying more
than nine separate times in a single semester.