What majors at what schools?
One example I know of is physics at Berkeley, which requires about 41% of the total credits for the BA degree. The prerequisite sequence is 6 semesters long for the courses required for the major, but some elective physics courses require 7 semesters to get to them. AP credit can shave off at most one semester from the prerequisite sequence, so the minimum prerequisite sequence is 5 semesters for the required courses, or 6 semesters if the student wants to take some of the electives. In addition, the Berkeley College of Letters and Science does not accept AP credit to fulfill its “seven course breadth requirement”.
Fortunately, for physics majors at Berkeley, the applicable credit unit ceiling does not come into affect unless one goes beyond 8 semesters, and it does not count AP or IB credit, or college credit earned before high school graduation against the credit unit ceiling.