This is low-yield material and you won't need to memorize this for the MCAT. The MCAT will not test you on the gray area and you should instead strive to understand the idea. In practice, there's no accepted value. If you're doing a distillation and simple distillation doesn't separate your materials well, you try fractional distillation.
What defines "significant" is open to interpretation. As the boiling points get closer, you need to increase the surface area in the distilling column, to give it more of a fractional nature. That is the concept that you need to understand.
However, if you want an exact number to cling to, most lab manuals list a 30˚C difference as the cutoff between simple and fractional. It's an arbitrary number that in all likelihood appeared in one lab manual many years ago that people blindly repeated in future books.