There is no rule.
The number of lessons depends on a number of different factors and all children are different. We can promise that we will never tell you to have additional lessons if we believe that we are unable to provide any further benefit for your child.
Your tutor will be able to advise on the progress of your child, and of course, you as a parent and your child will be able to judge if any further lessons are required.
We have customers that have been with us for over 3 yers having weekly lessons, and some for just a few months.