This course has a subtitle, META101x: Thinking about thinking. It sounds good because it catches one of the main ideas of critical thinking, that it is a kind of thinking directed to other thinking.
It is created in the University of Queensland by Deborah Brown and Peter Ellerton. According to the description the course is based on philosophy and it is primarily aimed on analysing and understanding arguments. Of course it isn’t the comprehensive course about critical thinking, it is more likely an introduction to the topic from a philosophical point of view. But still it may be good as a starting point of studying critical thinking.
It is a free course, you need to pay only for certificate, so you can try it yourself. It is also self-paced and authors estimate that if you are going to spend four weeks on this course you need from one to four hours each week. So in total you need from 4 to 16 hours to complete the course.