I took around 1 week for each exam, give or take a day or two, and I was studying for about 5 hours each day. I studied for them over the summer, during my break from university. I've never been able to skim over a book before answering a few questions and then be able to pass an exam.
It really depends if you like studying or not, and what the content is!
Personally, I don't like studying and I never have. I loved studying the PPL theory though!
Oh and after doing air law first (which is the logical choice) I'd say get your head in to the aircraft general. I did this last but wish I did it earlier. By actually knowing a little about the aircraft you fly, it will give you greater appreciation when doing power checks etc!