An interesting angle on this, relevant to the 18hrs figure, is whether ground school is included in the "course of study". I think it is.
It is feasible (not by much though) for a PPL or an IR to exceed 18hrs/week especially if ground school is included. On a PPL or an IR you are likely to fly twice a day, 6 days a week, which is 12hrs/week, but obviously one could go past that if not feeling too tired and there is no thing else to do in the area.
But most of the other "courses" in flying aren't going to get past 18hrs/week, especially if (as is most strongly advisable) you sit the ground exams before going to the USA.
As I say above, it is down to the school in the end.