We went to Kennedy first in 1991 and again this past December. The format of the tour had changed and we found it took longer the second time.
The first time, you got on a tour bus that made several stops. But, you kept getting back on the same bus and going to the next location. I remember the tour taking maybe 2 hours and then we were able to see some of the IMAX shows and exhibits at the Visitor's Complex (the area where you first walk in).
In December, the tour was set up differently. You took a bus to the first area of the tour and it dropped you off. You could spend as much or as little time at that stop as you wanted. Then, you had to line up and take the next bus to the next stop. The buses just kept rotating around and taking people between the three (or 4 - can't remember) tour stops. After the last stop, we ended up waiting 30 minutes for a bus to take us back to the Visitor's Center. We needed to be back at WDW for a 7:00 p.m. ADR so we did very little else before leaving. Plus, by the time we got back, most of the movies already had their last showings for the day.
My advice would be to read the schedule before getting on the tour. If you want to see any movies, see them before the tour. We also bought a combo ticket that would get us into the Astronaut's Hall of Fame - which is located down the road from Kennedy. We didn't end up having time to stop there so we shouldn't have spent the extra money on the ticket.