Cheers both,
I really had no idea how long to aim for, so thanks for the advice. My sup will be seeing drafts as I go along, but I was just thinking on the basis of the 2 that I've done that a lot of correcting and re-writing will need to be done. I think I might aim at 3 months; if I can do it, then great, if I don't I'll still have a good amount of time left.