Hi, I just got back and the most time consuming (but not to be missed) thing is the bus tour that takes you into Kennedy and the launch sites. They say it takes 2 to 2 1/2 hours for that tour. It definately took at at least 2 1/2 (although we were reading most of the signs, no kids on this trip).
Kennedy opens at 9:00, and the bus tours start at 10:00.
In addition to the tours there are numerous shows and museums/exhibits to look at. Kennedy offers different price options for what you want to do. For example you would not buy the combo because you wouldn't have time to the Astronaut Museum, IMAX, etc.
I would recomend looking at their web site and coming up with a plan and knowing you won't get to see it all.
It is really a remarkable place and gives you a prideful feeling.
For our part, we were there Saturday - low crowds, arrived at 9:40 left around 5:00 and did not even get to see the IMAX show! However, we had tickets to eat lunch with an Astronuat which was from 12:15 to 1:30. (So we were unable to do the bus tour until after the lunch.)
I hope this helps! Emjoy!
Lee Ann