Are you thinking about going to Walt Disney World? What is stopping you from starting your planning now? I'm serious when I say this! You want to go? Contact me and we can start discussing the pro's and cons of when to visit... etc..
But... to best answer the question, I'd say... AT LEAST 6 to months 9 months prior to arrival!
(There is one caveat. The following year's package prices don't come out until July, and packages can't be booked until the prices have become official (though you can reserve a room). For instance, 2015 package prices came out July of 2014. BUT this shouldn't stop you from starting the planning!)
Basically, you want your accommodations fairly settled (although all is modifiable and promotions can still be granted) before you hit your 180 day window (roughly 6 months). Shortly before this window, Walt Disney World's official park hours have been released for your month of vacationing, we can begin to estimate crowd trends (have ideas about which parks to tour on which days), and at 180 days, you can make your Advanced Dining Reservations as well as reservations for experiences, such as princess makeovers at Bibbidi Bobbidi Boutique and pirate transformations at Pirates League. Again, you can cancel all of these reservations at no charge within the cancellation guideline timeframe 24 hours to 2 weeks prior to the reservations depending on the reservation. Remember that even if you have included a dining plan in your package, reservations for your table service restaurants are still very, very, VERY strongly recommended.
Seriously, are you thinking about a Walt Disney World Vacation? As I've mentioned previously, it's only a refundable $200 down to book a package! The balance is due 45 days prior to your vacation (you can make payments however you wish, or pay the rest in total at 45 days), at which point it is still refundable, minus the $200.