You have 18 months from taking first to passing last exam.
I have one to go, and would rate the hardest as:
Air law: Lots of dry info to absorb
Navigation: Some complicated maths and daft exam questions
Meteorology: Lots to learn, but fascinating
Whilst there is not set order of taking them, you will probably find that the school you choose will insist on you passing certain exams before they allow you to do certain stages of training - like passing the Nav exam before letting you go cross country solo.
There's also an RT exam you have to pass before you can use a radio outside of being a student.