How many credit hours of mathematics does an average successful student take before applying to graduate school?
This tends to vary a lot, but here's what I'd consider reasonably good preparation for pure math graduate school in the United States:


Three semesters of calculus (possibly less if you've had calculus in high school)
A semester of linear algebra
Two semesters of abstract algebra
Two semesters of real analysis
One semester of complex analysis
One semester of topology
At least 2-3 more semesters of advanced electives.  These can vary a lot, but it helps to have some expertise in a few different subfields of math before attending graduate school.


So it seems that I'm recommending at least 12-13 semesters of mathematics.  Assuming 4 credit hours for a one semester course, this is 48-52 credit hours of college mathematics.