I wrote and revised the first chapter in a month, while the second and third chapters were completed in about 3 weeks. Revisions took another week. I had originally allotted a month for each chapter (2-3 weeks writing, 1-2 weeks revisions), but one of my advisors had to leave the country a month early and my defense had to take place before he left. The adrenaline helped.
About 10 weeks, though I was working part time as well, and could have done with another week or two for revision - I didn't have time to take a break between writing and revision which is something I find very helpful.