You can choose your lesson time between 30 min, 50 min or 2 x 30 min.
For adults I generally recommend 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 the best possible way whereas 50 minutes would be appropriate only in certain cases.
When children advance quickly it makes sense to upgrade to 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.