From a teacher's perspective, it really depends on the goal of the student. If a student has a specific question in mind and only wants to focus on that, then potentially 30 minutes is adequate. Also, for conversation practice, 30 minutes is enough time to have a conversation and therefore get in some useful language practice. Perhaps shorter lessons are also better suited to children or those with shorter attention spans.
However, if a student needs to develop their abilities across a range of skills and wants to work towards a bigger goal, then longer lessons are always advisable in my opinion. A new grammar point, for example, requires the introduction of the form and the uses, as well as then practice which is not possible to do adequately in a short time period.