Students who work with DSP are strongly urged to communicate with instructors as soon as possible. However, the University must make every feasible effort to accommodate students regardless of the notice.
A specific answer to this question is contextual with the nature of the needed accommodation. For example, a student who has a vision impairment may need to use a dark pen and write in large print. Perhaps s/he will need to write the "short answers" on a separate sheet of paper instead of in the blank spaces on the test document itself. This accommodation really doesn't need any advance preparation, so not much advance notice may be needed. On the other hand, if a student's accommodation requires an instructor or the department to arrange a separate, quiet room; a proctor; or both, then more time is obviously needed. For such matters, especially when a department must arrange a proctor, the proctoring office needs advance notice of two weeks for midterms and six weeks for finals. This means students should make sure that they give their professors enough time to contact the proctoring office.