I only know about the Open University in this respect. They estimate that student on a 60-point course (equiv. half a year at mainstream, f/t uni) will need to dedicate around 8-10 hours a week to it over 30 weeks. Obviously there are some courses and students that need more than this, but the joy is that you can pick your study hours to suit, and (depending on your degree subject) you can choose to take 30 point courses, which require less time commitment.