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.