* Course load: A full-time student might take 15 credits in one semester (or quarter, depending on the system). A part-time student might spread those credits over multiple semesters.
* Course intensity: Some courses require significantly more work than others, even if they carry the same credit weight. A lab science course will typically demand more time than a humanities lecture course.
* Student's learning style and pace: Some students learn and absorb material faster than others.
In short, it could take anywhere from one semester (or quarter) to several semesters or even years to complete 15 credits.