A vocal lesson should last from twenty to forty minutes. An hour of steady vocalizing tends to tire the average student and the last minutes accomplish nothing. The teacher also is under a constant strain, and an over-long lesson sometimes does more harm than good. If, however, the lesson is divided into two periods, one for vocalizing and the other for repertoire, an hour’s work is quite satisfactory. In this manner, both placement and repertoire advance together and the student and teacher do not tire of either. It is often best to divide an hour’s work into two distinct periods, one in the morning and another in the afternoon, or to allow a rest period of some duration during the lesson if it is continuous.
If a student tells you that he finds practicing mezzo voce fatiguing when studying repertoire, beware! The act of singing must be carried out correctly at all times; tone focus, relaxation, and breath control should eliminate strain.