In college, D to an A in a month. She let the class make-up whatever assignments they had missed for partial credit and I studied my ass off and aced the final.
Also, D to a B in one of the hardest management classes we had to take, that was really nice to get, because I needed it to pass and graduate.
Those were my fastest comebacks in school, both of those were in the span of 4-5 weeks. My best overall comeback was probably going from an F at the beginning of the semester after the first exam to an A at the end of the class in a statistics class.
I didn't like school much, so I basically did as little as possible to get by and then killed myself during the last part of the semester to pass everything. I never learned either, I can't think of too many semesters in college where I didn't use this method