know approximately how many hours people spent / feel should be spent studying ground school.
How long is a piece of string?
The answer to that depends partly on educational ability and partly on background interests. The course includes quite a substantial amount of information, covering quite a lot of subjects, and a lot of it will be new to anyone. A good grounding in the sciences will help with aircraft technical, meteorology and medicine, but air law involves a lot of rote learning. I actually found ground-school very satisfying though.
Which subjects would you suggest its most useful to take first and which leave to the end.
Air law because you need this for your first solo and because once you've got this out of the way, the rest will seem positively fun by comparison. Also learn the phonetic alphabet early because you can practice reading car number plates every day, and it takes a while to become fluent.
If you have the discipline, it wouldn't be a bad idea to do the whole of ground school prior to starting the course. However, I would sit some of the exams (e.g. nav) after getting a bit of practical experience.
Any recommendations or good / bad experiences with self study CDs
I bought the Oxford RT CD and was unimpressed. It didn't tell me anything I couldn't have learned from a much cheaper book, and I could think of far more imaginative forms of computer aided learning. I also found it very slow. Admittedly, I had wanted it more for the practical than the theoretical test.
I would suggest
AirQuiz - Online Practice Examinations for Pilots! - very reasonably priced; very responsive administrator and I gather it has just had a series of questions updated.