Students may wonder, when they begin private tutoring, how much time they need to "improve their skills". In the case of English, either writing, or improving their foreign language skills, I have found that 15-20 hours is a good initial target for both students and tutors. This is enough time to become familiar with the tutor's methods and style, to relax and get into a learning mode again, to compare a "before" and "after", to make a self-evaluation. For tutors, this should be enough time to move a perceptible degree in the field, not a whole "level", but enough to show the student that he can improve, and you can help him. It also shows the student how intense this work can be, and helps him decide if he wants to continue the process.
After about 20 hours, the student and tutor can discuss the progress to date, and any potential future work together. This is the time to review the original goals, any new or modified goals, a new or revised timetable or schedule, additional or new teaching resources (perhaps it's time to finally use a formal text), and methods of evaluation. I would suggest setting up 15-20 hour blocks of time, and evaluating progress after each block. In this way, the tutoring doesn't just "go on forever", with no clear idea of when it's "finished". Both students and tutors need to know when it's time to "end", when it's "enough", in order to make sure that the goals are met within that agreed time frame. Such blocks make the tutoring feel more structured, like a curriculum, and also allow the tutor to plan when to accept future students.