I'm planning a trip which is 18 months away! We have tentative plans and have done a lot of research so far and will continue to do so. These plans will probably change a few times before they are finalised as I learn more info and decide that I want to do more things. We will book the room probably 11 months before we travel and then book the air travel and rooms for the rest of the trip over christmas time (the air companies have sales on then) which will be 8 months before we go.
I love the whole planning stage, I guess I think that if I do a lot of research before hand if things don't go to plan then I have enough knowledge to make better judgements to go with the flow.
Happy planning.