Thinking that travel duration is solely defined by the budget, especially in the low season, forgets the reality of time constraints for a student.
Most of us do not have the luxury to choose between two or three weeks: it is the exams or the summer job that dictate the schedule.
So, if I go for two weeks in the low season, it is not just to save money, but because I do not have more available time before the return to school.
It is a matter of compromise: we look for the best timing for an experience that fits into our busy lives.
For example, I went to Rome for a week between two sessions, even if it was more expensive than two weeks in full low season elsewhere, because it was the only window available.