I worked full time and I completed my theory within 10 weeks. I targeted the major ones first and read the book once which took me about a week or just over for the big topics like AGK, NAV, MET, AIR LAW, and went on to confuser which helped me immensely and whilst it was fresh in my head I took the exam. It works and can be done if your willing to spend three hours each evening and full weekend. The small topics like comms, HP&L and mass and balance will probably take you about a week from when you start reading the book and finishing the exam.
Just dedicate any spare time towards your theory and once you complete them you will really enjoy the flying part bcos you would have one thing less to worry about.
good luck.