The number of hours of lessons you should take every week depends on your study time and your budget.  

Although your class time is limited, you can find ways to use it in a more efficient way.  For example, you can decide to do two-hour lessons every week.  If you take lessons face to face, you can take one to two-hour lessons for each session, whichever is most convenient for you.  You should also take your attention span into consideration, as you need to make sure that you are able to concentrate for the duration of the lesson.  If you take lessons online, I suggest that you take 30 minutes to 1 hour each time and not more.  Thus, you can do two or four times a week, which enables you to study more frequently and more efficiently.