If I can offer any advice, I would suggest you have a VERY clear structure before you start, plan each chapter and make sure each section has meaning.
My 2nd supervisor also said that you should be able to get a good idea of what the thesis is about, the problems you are solving and the results.. from simply reading the titles - so I'd suggest you sort out the titles and go from there.
It really is a 'how long is a peice of string' kind of question, however hopefully you'll not do what I did and 'write like a chav' for months, only to be told that your thesis reads like a Little Britain sketch (ok, so slightly exagerated.. but hey!)