You can choose your lesson time between 30 min, 40 min, 50 min or 2 x 30 min.
(For individual private lessons I only offer 30 or 50 min)
For teenagers and adults I generally recommend 40 or better 50 minutes.
For younger children (age 4-7) 30 minutes are mostly sufficient as concentration will wane after that period of time. 30 minutes twice a week is most recommendable for this age group, whereas 50 minutes would be appropriate only in certain cases. 40 minutes is kind of a sweet spot lesson time.
When children advance quickly it makes sense to upgrade to 40 or even 50 minutes or to the aforementioned 30 minutes twice a week.
Advanced students regardless of their age are well-advised with 50 minutes or 30 minutes twice a week.