Some students can benefit from tuition on an ad hoc basis, whilst others find it more productive to attend regular lessons to keep up with their school work.
For instance, some students have many other commitments such as Co-Curricular Activities that prevent them from adhering to a regular tuition schedule. These students may find it more beneficial to seek help in their academic subjects as and when necessary.
In my experience, Junior College (JC) and Polytechnic level students tend to approach me for tuition with the view of having lessons on a regular weekly basis. University students on the other hand, with their broader curricula and more varied expectations of what they hope to achieve out of private tuition, tend to approach me on an ad hoc basis (say, for a period of a couple of months, or occasionally just for several lessons before a final exam).
Sometimes, students would also try to cram as many tuition slots as possible nearer the examination season, though in my opinion that is not very advisable and it is more beneficial and less stressful to begin tuition early. It is critical for the learning process that doubts and conceptual difficulties are rectified early so that more time is available to fully master and appreciate the subject.
Whether to have regular tuition on a weekly basis or on an ad-hoc basis is thus up to individuals needs and expectations. There is no one size fits all formula. Tutors like me are prepared to be flexible and accommodating this regard.