Huh?
Since there are three quarters in an academic year, three quarters of N quarter hours is the same amount of class / credit as two semesters of N semester hours. If a student takes 1.5**N* quarter hours per quarter, s/he will complete as much class / credit in 2/3 the time (two quarters instead of three).
While N = 12 is typically full time for financial aid purposes, N = 15 or 16 is typically needed to graduate in four academic years (8 semesters or 12 quarters); sometimes high credit majors like engineering majors may require N > 16 to graduate in four academic years.