Obviously, you know how your kids handle things best. For my son (almost 5), he likes the countdown/anticipation - for him, the building excitement as a trip gets closer is part of the joy. I think he'd be disappointed if we surprised him with a trip, no advance notice. But at this age, too long a countdown is "FOREVER!!!" and would just lead to frustration for him and me both.
The last time we went (when he was 3 1/2), we told him a bit less than 3 weeks before. Then together, we made a paper chain (with Disney themed activities I'd pre-selected written inside), and we began our countdown. He was happy to tear off a chain link every morning, and the visible reminder of how long we had left helped keep him from asking me, every hour on the hour, how much longer until we went.
For our upcoming trip, he'll be 5. I intend to tell him about a month before. I'm sure he'll want to do a countdown again, and we'll do some Disney "research" together to choose must-do activities in the parks, brush up on a few favorite movies, do a few crafts, etc, to while away the days until our departure.
For us, that's what works. I hope you find the "right" time to tell your kids, and they enjoy the anticipation, too!
