There have been shows with 3+ year runs though.....
Endless Summer On Ice - 2007, 2008, 2009
The Nickelodeon Celebration Parade - 2004, 2005, 2006
Monster Rock which is maybe going into it's what 7th or 8th year?
and of course the almost 10 year run of Hot Blooded
But yes, it is true that most of the time a show is a 2-year run at Kings Island.