I've been there many times. You really do need a full day to experience it. The Shuttle Launch Experience is a great ride (simulator), there are two omnimax theaters, lots of shows and exhibits, and great food & shops.
Then there's the bus tour. That alone can take 3-5hrs, depending on how long you stay. The Apollo center alone can be a 2hr investment.
It's about a 45-60 min drive from the Orlando area, but if you have the chance to see it, DO IT!