Length of planning depends on your depth of planning.
Are you a Winger? Do you make reservations for air, car, and hotel, buy tickets, and call the trip planned? This kind of trip takes a day to plan.
Or are you a Monk? Do you plan out daily itineraries, use a touring plan, make 3 or 4 restaurant PSs daily, and ask things on the boards like "how many minutes does it take to get from Star Tours to the Indiana Jones Stunt show?" That kind of trip takes months to plan.
I'm a Winger; I am obsessive about my travel arrangements (wouldn't do to be stranded without accomodation or transportation), but I rarely know which park I'm going to be in on what day, so I can almost never make advance PSs and I have never followed a touring plan of any kind. If I had to, I could plan a Disney trip in a day, one day in advance. But the less advance notice you have, the less likely you are to get the bargains on air, car, and hotel.