It is NEVER too soon to start a conversation, as this answer is different for everyone.
For Walt Disney World and Disneyland I find that most clients book their trip AT LEAST 3 months in advance. Advanced Dining reservations open up 60 days before, so this gives us plenty of time to plan and strategize. (Although, I've had many clients use me to help book last minute trips who also had amazing vacations!)
When booking a Vacation Package at a Walt Disney World or Disneyland Resort you will have the peace-of-mind of knowing your $200 deposit and any monies paid are FULLY REFUNDABLE as long as you cancel 31 days or more before check in.
DISNEY CRUISE LINE clients can take comfort in their trips being FULLY REFUNDABLE 90-120 days before their sail date. (Depending on length of cruise.) For even more flexibility Disney offers a Vacation Protection Plan that can be added up until final payment date.
I have helped clients plan OVER A YEAR in advance, and with less than a week to go. The further out we plan the more options you will have. In general, clients seem to save a lot more money when booking at least 6 months in advance.