I'm thinking 2 semesters. A lot of that 'attrition' arises from students transferring during the summer after 2nd semester grades. Mathematically, it would be hard to survive 30 credits and somehow drop below the required GPA from 6 credits in the 3rd semester. What is the point you are trying to make?
Assuming arguendo that your saying a full year constitutes 3 semesters( Fall, Winter, and then Summer semesters). During the summer, my T4 only allows up to 7 credits to be taken.
It would not be 1/3rd of the total credits. Closer to 1/5th. The summer class schedule is more condensed and thus the 7 credit maximum limit. The fall/winter semesters you typically take 15 credits. Also, the more credits you earn the less impact a bad grade has. Thus bad grades arising out of the 3rd semester will be softened and therefore not much attrition shall arise. My T4 = UDM. The good thing about insane attrition is that it brings a sharp focus to the task at hand (lawyerly analysis on exams) . Even though academic attrition is mostly urban legend, it certainly exists here and keeps you scratching and clawing just to keep your head above water.