I guess it all depends on what you want to do. I started planning our first trip about 10 months out. I did this mainly because my boys were only going to be 4.5 yo and I wanted to make sure to avoid anything that would scare them.
Through this board and others, I learned that there are some things that you really have to do months in advance. For instance, if you want to do one of the illuminations cruises or breakfast at Cinderella's Castle you have to book them the moment they become available or take a big chance that you won't get to do it.
In my experience, most things can be done much closer to your travel date depending on how busy a time of year it will be. I'd make a list of the things that you really want to do, and make sure you get those set up. After that, you can slow down the pace if you want.