How many words does it need to be? Depending on that I've found it best to break down the period of time, so say the first X months spent researching and compiling your information, taking notes and working through the way in which you want your dissertation to flow.

Then set aside a few months for writing, and break each week/fortnight/month (whatever works best for you) into realistic word count goals. I'm a procrastinator with anxiety and this has worked for me, so hopefully this helps. Good luck! :)