Sorry Amy - didn't mean to scare you. My supervisors don't usually have the same opinions so I could ask another one and they'd probably say it was doable! I agree with Sue, your supervisor has set a very high bar for you. I really think this process is individual and can't be generalised between people with different fields, research, support, writing styles, levels of work already completed etc etc. There are so many variables it would be difficult to generalise, particularly when we don't know the intricacies of each others projects. I'm also doing chapters in the style of papers so they may take longer - if you are doing a different thesis style chapters may be quicker. I think 3 months is a tall order, but some people can do these things. I couldn't do that and will spend about 6 months writing my draft (along with quite a large extra piece of data collection); but that's me. You need to find what works for you, your own pace and work to that. Good luck!
:-)

You're right about it being different for each person and also for different disciplines, so maybe in my area what my supervisor is expecting is possible (if I keep telling myself that it might seem less daunting !:$ ). I guess all I can do is write at the pace that works for me, whatever that ends up being, and make sure I spend enough time on it that it's a decent body of work that reflects all the effort that's gone in to the project so far. My supervisor wouldn't want me to rush it just to get done in 3 months, he'd rather it was good, but he does still seem to think that 3 months is long enough to produce a good thesis. Hmm, I should stop stressing about how long it will take, and put more of my energies in to actually writing, and resign myself to it being finished when it's finished, whenever that may be! :-)