If you were simply going to say "On our next vacation we're going to Disney World," and they wouldn't drive you up a wall and they'd help plan, 3/4 of a year out might be okay. If you plan to make a big deal out of revealing the trip, I'd definitely do it closer to the date. I'd do a big reveal at Christmas if I had a trip planned for like February or something similar. But really I'd do it closer than 3 months. For a fall trip, you might have a "back to school" party with just your family
do something fun to end the summer, then announce the trip, give them each a Disney themed gift to give them something to look forward to and maybe some motivation to start the school year off well.
For my daughter's first trip, I put a huge display of Disney stuff (mostly princess stuff) on the couch and when she got home from a play date, I brought her in with her eyes closed until she was in front of it and then revealed it. In the middle was a note from Princess Tiana saying she was looking forward to meeting her. We left the next day.
With my child I can't do anything that will take her attention off of school. If we're going somewhere during the school year, I do a couple of week's notice if it isn't big like when we spent spring break at the beach. We went to Orlando last month for a Sea World trip for her fall break. She had a Thursday and Friday off and I picked her up from school an hour early that Wednesday. When she was walking to the van she said, "it would have been nice if we could take a vacation even though this break is short." When she closed her sliding door (we have a minivan) there was a sign that read "You are going to Sea World." That was how she found out. We went to the airport from there.
My daughter already knows about our trip in June, but that is mainly because there is a big cruise being planned that a lot of people from our church will likely be going on and I decided we would not be going. I don't want her to hear about it and think we aren't doing anything. I figured she might want to help plan but other than telling me she wants all the same character meals again and wants to dress up and be a princess, she won't talk about it much. When I mention it she will answer whatever question I have about what she wants to do when we go back but prefers not to talk about it because of how far away it is.