Some highly selective colleges restrict the number of AP or IB classes you can use for college credit, or don’t recognize any at all. My D1’s LAC limits you to the equivalent of 4 courses—a full load for one semester. Since they have graduation only once a year, and you pay a full year’s tuition regardless of how many classes you’re taking, that effectively means you’re there for four years. I suppose in principle you could use the 4 AP or IB credits to get through in three years by taking an extra-heavy load in the time you’re there, but I’ve never heard of anyone doing that. Most do just the opposite—the AP or IB credits allow them to take less than a full load some semesters.
The school places a strong emphasis on students staying for four years and graduating on time, with their class—and they’re quite successful at it. They have a very high graduation rate, and almost no one takes more than four years to graduate. And it becomes part of the student culture—students expect it, and seem to value it. Loyalty to one’s class is almost as strong as loyalty to the school.
Harvard currently grants credit for AP or IB only if you have the equivalent of 8 college courses, which can propel you into the Advanced Standing program. But beginning in 2020 they’re eliminating Advanced Standing and will give no credit for AP or IB. I can see their point: there are probably few if any AP or IB classes in which the content actually approximates that of a Harvard course in the same subject.
Most colleges grant credit for AP, IB, and dual enrollment much more liberally. I’ve known plenty of kids who have used those credits to get through our public flagship in 3 or 3.5 years or sometimes even less, often with encouragement from their parents who see it as a shrewd money-saving strategy.