Short answer
Set time limits from measured demand, job-length distribution, instructional need, and available staffed capacity. A fair policy may combine a maximum slot, a weekly per-user cap, protected course blocks, and an exception path for research or disability-related needs. Keep short-test access separate from long production time so students can still iterate.
Publish what counts toward a quota, whether setup and cleanup are included, how unused time is released, and how exceptions are evaluated. Audit the results by course, user group, and wait time. Judge fairness by who actually gets machine time, not only by equal minutes, and rely on published criteria and periodic data review rather than informal judgment calls.