I would be basing this decision on two things - did the mastitis clear up readily within a couple of days or did it linger a bit, and also - is this one in a series of mastitis episodes? For a once-off that responded quickly, I would just take the one course. If it took several days to improve or it's part of a repeated pattern of mastitis I would take the extra course.

You could of course take the middle road, finish after the one course then keep the script and use it if you have a repeat episode in the near future.