I've started a few weeks ago and already my supervisor wants me to start writing; he says I may not keep all I write now, but its better to start when you can and get things moving
I haven't completed all my research (social science), but I've been told that it's best not to think about your PhD as split between research and writing up because as you write you will identify new areas or new arguments that need to be addressed, which may involve going back and re-assessing your research. Not quite the same with the sciences, but I can still see where it would help you to start writing if you feel you can