We are using it in high school (junior years), and it is hard to fit the program in the timetable. What should the minimum number of lessons be per week to be effective in the long run?

We do have a pacing chart provided in your teaching guides to help you to plan your lessons. Generally, you would complete one five-part lesson in a week. This would help you complete the entire reading level within a year. If you have time limitations, you may allocate one part of the lesson for a week, however, students would not be finished within the year that way. If you have really limited resources, and do not have enough time to complete the reading level, you may only want to find out their weakest strategies so you can allow the student to work on them as a priority.