At my studio, I only offer 30 minute lessons to pre-Grade 1 (early beginner) students. Once Grade 1 standard has been reached, lessons are either 45 minutes or 60 minutes depending on age and ability.
But why? In the UK, 30 minute lessons are normal. It fit's in nicely with the hourly schedule so that if a student discontinues lessons or reschedules, it's very easy to slot another student's lesson in. However trying fit in each 30 minute lesson: scales, arpeggios, technical exercises, current repertoire, sight-reading, aural, theory, improvisation, composition, duet playing and other activities is very demanding. From a personal viewpoint as a teacher, it is optimal to cover all of these in each lesson.
Having a wide variety of activities develops the numerous skillsets of a musician, in order to provide a well-rounded experience for students. Learning music needs space and time. Condensing content into an intense 30 minute session can have a negative affect on independent learning, motivation and hence progress. There is not enough 'breathing room' for new concepts to take root, for musicality to be explored nor repertoire to be polished. Realistically, it only allows for one topic to be covered in depth whilst other skill areas may be quickly brushed over or left until short-notice before an exam.
None of these points bode well to craft an environment where students feel equipped with the key tools to take control of their practise at home. And it can result in a loss of interest and motivation as they move towards more depending music.
What are the benefits of a 45 minute lesson?
Review all pieces that have been practiced during the week
Learn new pieces, technical skills, and work on sight-reading
Plan what is to be practiced during the upcoming week
Work on composition and music theory
Do fun stuff e.g. improvisation, games, aural
Build healthy rapport between student and teacher
Learning music and learning to play an instrument is similar the process of learning a language.