Alternative Text

Is there a corresponding rule that an alt text must be within a certain limit of characters.
 
I used to use the advice, that alt text needs to be short. However, that is too vague. It does not describe how show or long the text can be.
 
WAVE has a limit that an alt text must be less than 100 characters. However, Cynthia Says has a rule that an alt text must be greater than 7 charters and less than 81 characters (including spaces).
 
Does WCAG 2.0 have a rule that clarifies how short or long an alt text can or cannot be until the developers need to use the longesc tag?