How far out you plan depends on what's important to you.
I've already started planning for our November 2017 trip... and our November 2019 trip!
As has been noted, there are a few benchmark dates you'll want to consider. Counting backwards, the big dates are 60 days out (when you can book your FastPasses) and 180 days out (when you can book dining --and premium restaurants will go quickly). I like to have my hotel reservations made at least a few weeks prior to the 180 day mark so the dining reservations can be easily linked.
Assuming that seems reasonable to you, the next questions are "How much do you now about Disney?" and "How much of a say do you want to give other members of your party?" There are so many hotels that it can take a week or two to read up on all of them and get your head around things - even longer if you're going to educate others.
If I were in your shoes, I would probably look to book my hotel in March, which means I'd want to know the exact dates if the trip as well as the hotel I'm booking early in the month. Perhaps I'd get Disney guide books as Christmas presents for those who might participate in the decision making, then set up some time a month or two later to talk and finalize things.
But I'm super-organized about the Disney planning.

If you have some flexibility around hotel and dining, you can probably start planning 3-4 months out with minimal trouble.
Good luck! Have fun!