I've been trying for 3 weeks to get a PS for the Princess Storybook Breakfast for the second week in October. It's always full. Just for fun, I started asking for Cinderella's Royal Table for the same time frame--and Guess What!?!? I got a PS for Cindy's and still no opening at the Storybook.
I'm wondering if people are more apt to cancel Cindy's PS because they have left a credit card deposit. With nothing to lose, people may not cancel the Storybook breakfast as often, even if their plans change. The problem is that you cannot just walk up to the storybook breakfast to get a seating--you have to have a PS to get into the World Showcase before 11am.
At any rate I would call as soon as you know your travel dates. The Storybook breakfast has proven to be harder for me to book. Like marn913 said you can book at 120 days out.
Good Luck!