I found that 2 weeks was sufficient to read one of the Trevor Thom books and do the sample questions in the PPL confuser.
I hadn't read the particular book on aircraft parts, systems etc (can't remember the title) but did the exam after reading the relevant section in the PPL confuser an hour before - I scored 85 percent.
The subject material is not too difficult but there's a lot of it. You'll need to do Air Law before your first solo.
Hope this helps.