Depends.
Do you have "the fear", LRD? 
I fannied about for 3 years not writing my PhD - just doing enough to look active, IYSWIM.
Then it was July, and I needed to be finished by September (getting threatening letters from my dept.).
So, inspired by "the fear" TM
I pretty much wrote my PhD from start to finish in about 3 months. I went underground, basically, only emerging to eat and sleep.
It was brutal, but I did it.
I think that I'd been brewing the ideas in my head for so long, it was sort of like laying a particularly painful egg, or something. Everything came out fully formed, and I didn't need to re-draft that much (thank god).
That said, my otherwise-delightful examiners did mention the high number of typos in their comments...
Clearly, I did not leave enough time to proof-read that well...
Friends, though, were much more grown-up, and responsible, and produced chapters in a timely fashion throughout, then polished up for the last 6 months or so. Like the spectacular hypocrite that I am, this is how I now tell my own students to work...
Not sure that this is very helpful, other than - I hope - to reassure you that you will get there in your own way. 