Hi LostOne,
I also did a systematic review for my PhD. It took me ages to review all the papers, and even longer to extract all the data manually to perform the meta-analysis. I also did the methodology chapter simultaneously, as I was critically analysing the results based on the methods used. So, in the same period I also decided on my methodology and equipment.
I was clueless on my topic when I started the PhD, as it was overlapping with three disciplines- so I had to cover the basics in all new areas. I would say it took me 7 months until I managed to get the first draft out, and on that draft I was working for another 2 months (interrupted) to polish it. It got published with very minor revisions, and also gave me the foundation to write my discussion chapter. In retrospective, I would say, it was time well spent. Also gave me an advantage compared with the other students who started fieldwork right away without really knowing what they are looking for.
The only drawback is that after four years, I had to update it... but I only added a few studies, I really don't have the time anymore to re-do all the graphs and tables...