I had friends who found their S-2 deferments withdrawn as soon as they completed undergraduate study.
In fact, in my part of the country (Georgia and South Carolina) you received an S-2 deferment for only eight semesters. If you couldn't finish college in that period of time, then the deferment ended; there were to my knowledge no exceptions.
Those going to medical school could receive an additional deferment (I heard the same story about ministerial study but have no personal knowledge of its veracity) but a deferment for any other sort of graduate study was out of the question.
I have no knowledge of the workings of draft boards in other areas of the country.
I personally think that a four year deferment is good enough.