I would allow at least one month per Chapter. It really depends on how long you expect they'll need to be, and the content of each Chapter. So for me, the theory part/literature review Chapters have been the most difficult whilst the methodology and empirical Chapters have been most straightforward; likewise, more or less time has been spent on them. I know for others it is the other way round! I would also look at other theses in your field to get a rough idea of length of each Chapter. I did this and from that could work out how long it would take me approximately, to get a rough draft. At full steam I can write a maximum of 1,000 words a day, but this is not good quality! So a 500 words per day target might be more realistic. BTW I'm writing my last Chapter now, the discussion, and I can say that I have written up the majority of my Thesis (79k so far) over the past 8 months. Good luck!