The Southern Association of Colleges and Schools (SACS) that accredits colleges/universities in the Southeast (and I'm sure it's similar in other parts of the country) has recently mandated a minimum of 120 semester hours to graduate. Some professional programs (nursing, education, accounting, to name a few) may require more. Divide that by 8 and you get 15 hours per semester. 12 hours is considered full time by every school with which I'm familiar. That being said, in the state of GA, because students are afraid of losing their HOPE Scholarships from the state, they typically sign up for 15 hours, but drop down to 12 so they can throw out the most difficult course and maximize their GPA's to keep HOPE. Many students make up the deficit by AP credit or joint enrollment credit from high school and are able to graduate in 8 semesters. However, if a student transfers or changes majors, it is unlikely that s/he will graduate in 8 semesters unless s/he attends summer school. Yes, a few courses are four-hour courses, but this doesn't make up the difference. At GT they refer to students as first-year, second-year, ..., fifth-year because just because a student has completed two years doesn't mean s/he has achieved junior status (typically 60 hours) due to taking the minimum full-time load.
We discussed this phenomenon at yesterday's faculty meeting, commenting that "back in the day," we picked a major and stuck with it. That's not typically the case these days, and we even have trouble getting students to declare a major by the end of their junior year!