I think it depends on how thorough you want to be with it and how long you can study before you lose interest or get distracted by less boring things. As an example I spent about 3 weeks reading through the electrics book after work and completing all the questions only to discover there were only 3 questions on electrics in the AGK exam, and I could've answered all of them without reading the book! No joke.
If you just skim over the books and then spend most of your time on the question bank it would no doubt be do-able in 6 months studying part-time.