I'm going to be the voice of opposition & say that every year is too often if you're not doing any other vacations throughout the rest of the year. There is more to the world than Disney & I believe kids should experience as much of it as possible.
We took our kids to
Disneyland when they were little (2 & 5), did a cruise last year & plan on taking them to WDW in probably 2009 (our last chance to go off-season). But we've also visited a dormant volcano in New Mexico, seen Shamu in San Diego & San Antonio, petted a hissing cockroach in Victoria, BC, Canada and this summer we're taking a bite out of the Big Apple.
I grew up vacationing with my family every year. I saw the Constitution, Mt Rushmore, Gettysburg, The Grand Canyon & many other historical & natural wonders. When we talked about the Declaration of Independence it wasn't just something I read about in a book because I had seen it. I better understood the scope of the Grand Canyon because I had seen it. My world was greatly expanded beyond my tiny little town because I had traveled.