Here are the answers to your questions below:
(a) How much time does one have to spend on the course (per week/month)?
: There are no restrictions since this is a self-study course. You can spend as much or as little time as you want each week working on the course materials. See Wendy's post
(b) I am not so much interested in learning how to paint with acrylics or water color. Oil paint is my thing. Do I have to go through lessons with other materials?
: No, you don't have to go through any lessons with other media. You can skip any parts that reference watercolor and acrylics. In any case, more than 95% of the course is independent of the media used. In my opinion specific media techniques play a relatively small role in the quality of a studen't painting. The key to making good paintings is to have a solid understanding of the underlying foundational principles that create great art. That is where most people are lacking in understanding. You can produce a masterpiece with a tin of house paint and a wooden stick if you fully understand these principles. Again, Wendy allude's to this in her post above
(c) Do you get fees back?
: Quite a few students after going through the full program and working hard at it, have seen big improvements in their work, and have been able to sell their paintings. Some have even turned professional and got into professional galleries. In that way they have recuperated the cost of the course many times over.
Is there a kind of critique? Is there any form of communication with the teacher(s)?
: Yes and no. At the moment, individual coaching, usually for a critique of a painting, is available on a very limited basis at an hourly rate Please note that this is not included in the cost of the course, since the basic price of the self-study course is very low (each of the 55 courses in the program works out to less than $10 each if you buy the full program in one go). However I do from time to time offer personal one-on-one coaching on a limited basis. I am a full-time professional painter, not a full-time teacher, and I don't hire instructors because I can't control the quality that way. I also offer two few week-long workshops each year in Tuscany for those students who would like personal feedback and teaching.
I hope this helps to answer your questions. If not please don't hesitate to ask for more clarification.