When is a 方言 a language and not a dialect
I'm going to assume that the question you want to ask are how mutually intelligible various 方言 actually are. This 2009 study by Chaoju Tang and Vincent J. Van Heuven has some data on it.

Here's the excerpted table on how mutually intelligible various 方言 are:

Word Classification:
 

and 

Word Understanding:


You can read more on the methodology in the paper itself.

For comparison's sake this 1989 John B. Jensen study found that Spanish and Portuguese are 


  50-60% mutually intelligible at least as measured on...passive
  listening to electronically re-produced voices