3-6 weeks per book should be enough part-time. If you're using the Thom books, do the questions as you go and sort out the stuff you didn't quite get the first time, before proceeding. At the end of the subject move onto the questions in the PPL Confuser as they are more or less identical to the real ones.
Personally I found the PPL books got easier, Air Law was the most boring and Met was the least clear, after that it was dead simple.